diff --git a/duration_form.php b/duration_form.php
index b09ac83..e0083e5 100644
--- a/duration_form.php
+++ b/duration_form.php
@@ -29,7 +29,7 @@ class mod_attforblock_duration_form extends moodleform {
 		
         $mform->addElement('hidden', 'ids', $ids);
        	$mform->addElement('hidden', 'id', $cm->id);
-        $mform->addElement('hidden', 'action', 'changeduration');
+        $mform->addElement('hidden', 'action', att_sessions_page_params::ACTION_CHANGE_DURATION);
         
         $mform->setDefaults(array('durtime' => array('hours'=>0, 'minutes'=>0)));
 		
diff --git a/lang/en/attforblock.php b/lang/en/attforblock.php
index a42d284..f94e2bc 100644
--- a/lang/en/attforblock.php
+++ b/lang/en/attforblock.php
@@ -110,6 +110,7 @@ $string['noofdayslate'] = 'No of days late';
 $string['noofdayspresent'] = 'No of days present';
 $string['nosessiondayselected'] = 'No Session day selected';
 $string['nosessionexists'] = 'No Session exists for this course';
+$string['nosessionsselected'] = 'No sessions selected';
 $string['notfound'] = 'Attendance activity not found in this course!';
 $string['olddate'] = 'Old date';
 $string['period'] = 'Frequency';
@@ -121,6 +122,7 @@ $string['resetdescription'] = 'Remember that deleting attendance data will erase
 $string['resetstatuses'] = 'Reset statuses to default';
 $string['restoredefaults'] = 'Restore defaults';
 $string['save'] = 'Save attendance';
+$string['session'] = 'Session';
 $string['session_help'] = 'Session';
 $string['sessionadded'] = 'Session successfully added';
 $string['sessionalreadyexists'] = 'Session already exists for this date';
diff --git a/lang/ru/attforblock.php b/lang/ru/attforblock.php
index 3215395..61f0c09 100644
--- a/lang/ru/attforblock.php
+++ b/lang/ru/attforblock.php
@@ -96,6 +96,7 @@ $string['noofdayslate'] = 'Опозданий';
 $string['noofdayspresent'] = 'Вы присутствовали';
 $string['nosessiondayselected'] = 'Не выбран день занятия';
 $string['nosessionexists'] = 'В данном курсе отсутствуют занятия. Сначала добавьте их';
+$string['nosessionsselected'] = 'Не выбрано ни одного занятия';
 $string['notfound'] = 'Элемент курса - \"Посещаемость\" не найден в данном курсе!';
 $string['olddate'] = 'Старая дата';
 $string['period'] = 'Периодичность';
diff --git a/locallib.php b/locallib.php
index 2f5721b..7aad9a1 100644
--- a/locallib.php
+++ b/locallib.php
@@ -329,9 +329,14 @@ class attforblock {
 
     private $currentgroup;
 
-    private $sessioninfo;
 
     private $statuses;
+
+    // Cache
+
+    // array by sessionid
+    private $sessioninfo = array();
+
     // arrays by userid
     private $usertakensesscount = array();
     private $userstatusesstat = array();
@@ -480,7 +485,13 @@ class attforblock {
                 'cgroup'    => $this->get_current_group());
         $sessions = $DB->get_records_select('attendance_sessions', $where, $params, 'sessdate asc');
         foreach ($sessions as $sess) {
-            $sess->description = file_rewrite_pluginfile_urls($sess->description, 'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $sess->id);
+            if (empty($sess->description)) {
+                $sess->description = get_string('nodescription', 'attforblock');
+            }
+            else {
+                $sess->description = file_rewrite_pluginfile_urls($sess->description,
+                        'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $sess->id);
+            }
         }
 
         return $sessions;
@@ -814,12 +825,36 @@ class attforblock {
     public function get_session_info($sessionid) {
         global $DB;
 
-        if (!isset($this->sessioninfo))
-            $this->sessioninfo = $DB->get_record('attendance_sessions', array('id' => $sessionid));
-            $this->sessioninfo->description = file_rewrite_pluginfile_urls($this->sessioninfo->description,
-                        'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $this->sessioninfo->id);
+        if (!array_key_exists($sessionid, $this->sessioninfo))
+            $this->sessioninfo[$sessionid] = $DB->get_record('attendance_sessions', array('id' => $sessionid));
+            if (empty($this->sessioninfo[$sessionid]->description)) {
+                $this->sessioninfo[$sessionid]->description = get_string('nodescription', 'attforblock');
+            }
+            else {
+                $this->sessioninfo[$sessionid]->description = file_rewrite_pluginfile_urls($this->sessioninfo[$sessionid]->description,
+                            'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $this->sessioninfo[$sessionid]->id);
+            }
+
+        return $this->sessioninfo[$sessionid];
+    }
+
+    public function get_sessions_info($sessionids) {
+        global $DB;
+
+        list($sql, $params) = $DB->get_in_or_equal($sessionids);
+        $sessions = $DB->get_records_select('attendance_sessions', "id $sql", $params, 'sessdate asc');
 
-        return $this->sessioninfo;
+        foreach ($sessions as $sess) {
+            if (empty($sess->description)) {
+                $sess->description = get_string('nodescription', 'attforblock');
+            }
+            else {
+                $sess->description = file_rewrite_pluginfile_urls($sess->description,
+                            'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $sess->id);
+            }
+        }
+        
+        return $sessions;
     }
 
     public function get_session_log($sessionid) {
@@ -890,7 +925,7 @@ class attforblock {
 
         foreach ($userids as $userid) {
             $grades[$userid]->userid = $userid;
-            $grades[$userid]->rawgrade = $this->get_user_grade($userid) / $this->get_user_max_grade($userid) * 100;
+            $grades[$userid]->rawgrade = calc_user_grade_percent($this->get_user_grade($userid), $this->get_user_max_grade($userid));
         }
 
         return grade_update('mod/attforblock', $this->course->id, 'mod', 'attforblock',
@@ -922,11 +957,40 @@ class attforblock {
                 'edate'     => $this->pageparams->enddate);
         $sessions = $DB->get_records_sql($sql, $params);
         foreach ($sessions as $sess) {
-            $sess->description = file_rewrite_pluginfile_urls($sess->description, 'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $sess->id);
+            if (empty($sess->description)) {
+                $sess->description = get_string('nodescription', 'attforblock');
+            }
+            else {
+                $sess->description = file_rewrite_pluginfile_urls($sess->description,
+                        'pluginfile.php', $this->context->id, 'mod_attforblock', 'session', $sess->id);
+            }
         }
 
         return $sessions;
     }
+
+    public function delete_sessions($sessionsids) {
+        global $DB;
+
+        list($sql, $params) = $DB->get_in_or_equal($sessionsids);
+        $DB->delete_records_select('attendance_log', "sessionid $sql", $params);
+        $DB->delete_records_list('attendance_sessions', 'id', $sessionsids);
+
+        // TODO: log
+    }
+
+    public function change_sessions_duration($sessionsids, $duration) {
+        global $DB;
+
+        $now = time();
+        $sessions = $DB->get_records_list('attendance_sessions', 'id', $sessionsids);
+        foreach ($sessions as $sess) {
+            $sess->duration = $duration;
+            $sess->timemodified = $now;
+            $DB->update_record('attendance_sessions', $sess);
+        }
+        // TODO: log
+    }
 }
 
 
@@ -1013,19 +1077,26 @@ function get_user_courses_attendances($userid) {
     return $DB->get_records_sql($sql, $params);
 }
 
+function calc_user_grade_percent($grade, $maxgrade) {
+    if ($maxgrade == 0)
+        return 0;
+    else
+        return $grade / $maxgrade * 100;
+}
+
 function update_all_users_grades($attid, $course, $context) {
     global $COURSE;
 
     $grades = array();
 
-    $userids = get_enrolled_users($context, 'mod/attforblock:canbelisted', 0, 'u.id');
+    $userids = array_keys(get_enrolled_users($context, 'mod/attforblock:canbelisted', 0, 'u.id'));
 
     $statuses = get_statuses($attid);
     foreach ($userids as $userid) {
         $grades[$userid]->userid = $userid;
         $userstatusesstat = get_user_statuses_stat($attid, $course->startdate, $userid);
         $usertakensesscount = get_user_taken_sessions_count($attid, $course->startdate, $userid);
-        $grades[$userid]->rawgrade = get_user_grade($userstatusesstat, $statuses) / get_user_max_grade($usertakensesscount, $statuses) * 100;
+        $grades[$userid]->rawgrade = calc_user_grade_percent(get_user_grade($userstatusesstat, $statuses), get_user_max_grade($usertakensesscount, $statuses));
     }
 
     return grade_update('mod/attforblock', $course->id, 'mod', 'attforblock',
diff --git a/renderer.php b/renderer.php
index db5128c..2c9532a 100644
--- a/renderer.php
+++ b/renderer.php
@@ -185,9 +185,9 @@ class mod_attforblock_renderer extends plugin_renderer_base {
             $table->data[$sess->id][] = $sess->groupid ? $sessdata->groups[$sess->groupid]->name : get_string('commonsession', 'attforblock');
             $table->data[$sess->id][] = $dta['date'];
             $table->data[$sess->id][] = $dta['time'];
-            $table->data[$sess->id][] = empty($sess->description) ? get_string('nodescription', 'attforblock') : $sess->description;
+            $table->data[$sess->id][] = $sess->description;
             $table->data[$sess->id][] = $dta['actions'];
-            $table->data[$sess->id][] = html_writer::checkbox('sessid', $sess->id, false);
+            $table->data[$sess->id][] = html_writer::checkbox('sessid[]', $sess->id, false);
         }
 
         return html_writer::table($table);
@@ -240,8 +240,9 @@ class mod_attforblock_renderer extends plugin_renderer_base {
                 get_string('hiddensessions', 'attforblock').': '.$sessdata->hiddensessionscount);
 
         if ($sessdata->perm->can_manage()) {
-            $options = array('deleteselected' => get_string('delete'),
-                    'changeduration' => get_string('changeduration', 'attforblock'));
+            $options = array(
+                        att_sessions_page_params::ACTION_DELETE_SELECTED => get_string('delete'),
+                        att_sessions_page_params::ACTION_CHANGE_DURATION => get_string('changeduration', 'attforblock'));
             $controls = html_writer::select($options, 'action');
             $attributes = array(
                     'type'  => 'submit',
@@ -292,7 +293,7 @@ class mod_attforblock_renderer extends plugin_renderer_base {
         $sessinfo = $date.' '.$time;
         $sessinfo .= html_writer::empty_tag('br');
         $sessinfo .= html_writer::empty_tag('br');
-        $sessinfo .= empty($sess->description) ? get_string('nodescription', 'attforblock') : $sess->description;
+        $sessinfo .= $sess->description;
 
         return $sessinfo;
     }
@@ -608,7 +609,7 @@ class mod_attforblock_renderer extends plugin_renderer_base {
             $row->cells[] = $i;
             $row->cells[] = userdate($sess->sessdate, get_string('strftimedmyw', 'attforblock'));
             $row->cells[] = $this->construct_time($sess->sessdate, $sess->duration);
-            $row->cells[] = empty($sess->description) ? get_string('nodescription', 'attforblock') : $sess->description;
+            $row->cells[] = $sess->description;
             if (isset($sess->statusid)) {
                 $row->cells[] = $userdata->statuses[$sess->statusid]->description;
                 $row->cells[] = $sess->remarks;
diff --git a/sessions.php b/sessions.php
index 54314be..570d62f 100644
--- a/sessions.php
+++ b/sessions.php
@@ -44,7 +44,6 @@ switch ($att->pageparams->action) {
                 redirect($url, get_string('sessionadded','attforblock'));
             }
         }
-
         break;
     case att_sessions_page_params::ACTION_UPDATE:
 		$sessionid	= required_param('sessionid', PARAM_INT);
@@ -63,6 +62,92 @@ switch ($att->pageparams->action) {
             redirect($att->url_manage(), get_string('sessionupdated','attforblock'));
         }
         break;
+    case att_sessions_page_params::ACTION_DELETE:
+		$sessionid	= required_param('sessionid', PARAM_INT);
+		$confirm    = optional_param('confirm', NULL, PARAM_INT);
+
+        if (isset($confirm)) {
+            $att->delete_sessions(array($sessionid));
+            update_all_users_grades($att->id, $att->course, $att->context);
+            redirect($att->url_manage(), get_string('sessiondeleted','attforblock'));
+        }
+
+        $sessinfo = $att->get_session_info($sessionid);
+
+        $message = get_string('deletecheckfull', '', get_string('session', 'attforblock'));
+        $message .= str_repeat(html_writer::empty_tag('br'), 2);
+        $message .= userdate($sessinfo->sessdate, get_string('strftimedmyhm', 'attforblock'));
+        $message .= html_writer::empty_tag('br');
+        $message .= $sessinfo->description;
+
+        $params = array('action' => $att->pageparams->action, 'sessionid' => $sessionid, 'confirm' => 1);
+
+        echo $OUTPUT->header();
+        echo $OUTPUT->heading(get_string('attendanceforthecourse','attforblock').' :: ' .$course->fullname);
+        echo $OUTPUT->confirm($message, $att->url_sessions($params), $att->url_manage());
+        echo $OUTPUT->footer();
+        exit;
+    case att_sessions_page_params::ACTION_DELETE_SELECTED:
+		$confirm    = optional_param('confirm', NULL, PARAM_INT);
+
+        if (isset($confirm)) {
+    		$sessionsids = required_param('sessionsids', PARAM_ALPHANUMEXT);
+            $sessionsids = explode('_', $sessionsids);
+
+            $att->delete_sessions($sessionsids);
+            update_all_users_grades($att->id, $att->course, $att->context);
+            redirect($att->url_manage(), get_string('sessiondeleted','attforblock'));
+        }
+
+		$fromform = data_submitted();
+        // nothing selected
+        if (!isset($fromform->sessid))
+            print_error ('nosessionsselected','attforblock', $att->url_manage());
+
+        $sessionsinfo = $att->get_sessions_info($fromform->sessid);
+
+        $message = get_string('deletecheckfull', '', get_string('session', 'attforblock'));
+        $message .= html_writer::empty_tag('br');
+        foreach ($sessionsinfo as $sessinfo) {
+            $message .= html_writer::empty_tag('br');
+            $message .= userdate($sessinfo->sessdate, get_string('strftimedmyhm', 'attforblock'));
+            $message .= html_writer::empty_tag('br');
+            $message .= $sessinfo->description;
+        }
+
+        $sessionsids = implode('_', $fromform->sessid);
+        $params = array('action' => $att->pageparams->action, 'sessionsids' => $sessionsids, 'confirm' => 1);
+
+        echo $OUTPUT->header();
+        echo $OUTPUT->heading(get_string('attendanceforthecourse','attforblock').' :: ' .$course->fullname);
+        echo $OUTPUT->confirm($message, $att->url_sessions($params), $att->url_manage());
+        echo $OUTPUT->footer();
+        exit;
+    case att_sessions_page_params::ACTION_CHANGE_DURATION:
+		$fromform = data_submitted();
+        $slist = isset($fromform->sessid) ? implode('_', $fromform->sessid) : '';
+
+        $url = $att->url_sessions(array('action' => att_sessions_page_params::ACTION_CHANGE_DURATION));
+        $formparams['ids'] = $slist;
+		$mform = new mod_attforblock_duration_form($url, $formparams);
+
+	    if ($mform->is_cancelled()) {
+	    	redirect($att->url_manage());
+	    }
+
+        if ($mform->is_submitted()) {
+            $formdata = $mform->get_data();
+
+            $sessionsids = explode('_', $fromform->ids);
+            $duration = $formdata->durtime['hours']*HOURSECS + $formdata->durtime['minutes']*MINSECS;
+            $att->change_sessions_duration($sessionsids, $duration);
+            redirect($att->url_manage(), get_string('sessionupdated','attforblock'));
+        }
+        
+        if ($slist === '')
+            print_error ('nosessionsselected','attforblock', $att->url_manage());
+
+        break;
 }
 
 $output = $PAGE->get_renderer('mod_attforblock');