diff --git a/classes/task/grade_participation_task.php b/classes/task/grade_participation_task.php
index 6dae565..f859ff8 100644
--- a/classes/task/grade_participation_task.php
+++ b/classes/task/grade_participation_task.php
@@ -44,17 +44,17 @@ class grade_participation_task extends \core\task\scheduled_task {
         $palestras = $DB->get_records_sql("
             select p.id, p.course, p.name, p.startdate, p.duration, p.checkinterval
             from {palestra} p
-              inner join {palestra_presence} pp on pp.palestraid = p.id
               inner join {course} c on c.id = p.course
             where
               (p.needsupdate = 1) and
               (c.visible = 1) and
-              ((c.startdate <= ?) and (c.enddate = 0 or c.enddate >= ?))
-              and ((p.startdate + (p.duration*60)) < ?)
-            group by p.id, p.course, p.name, p.startdate, p.duration, p.checkinterval
-            having max(pp.lastcheck) < ? - p.checkinterval*60
-
-        ", [$now, $now, $now, $now]);
+              ((c.startdate <= ?) and (c.enddate = 0 or c.enddate >= ?)) and
+              (select count(*)
+               from {palestra_presence} pp
+               where pp.palestraid = p.id and
+               (pp.lastcheck+(p.duration*60)) < ?
+              ) > 0
+        ", [$now, $now, $now]);
 
         mtrace(count($palestras)." palestras to grade...");
 
diff --git a/grade.php b/grade.php
index 63920e1..a9073c0 100644
--- a/grade.php
+++ b/grade.php
@@ -26,6 +26,7 @@
 
 require_once(__DIR__ . '/../../config.php');
 require_once("lib.php");
+require_once($CFG->libdir."/gradelib.php");
 
 $id = required_param('id', PARAM_INT);
 $userid = optional_param('userid', 0, PARAM_INT);
diff --git a/lib.php b/lib.php
index 9f061aa..e0b3e40 100644
--- a/lib.php
+++ b/lib.php
@@ -207,10 +207,12 @@ function palestra_update_grades($palestra, $userid=0, $nullifnone=true) {
     global $CFG, $DB;
     require_once($CFG->libdir.'/gradelib.php');
 
+    $now = time();
+
     if ($userid) {
-        $presences = $DB->get_records("palestra_presence", array("palestraid"=>$palestra->id, "userid"=>$userid), 'userid, starttime, lastcheck');
+        $presences = $DB->get_records("palestra_presence", array("palestraid"=>$palestra->id, "userid"=>$userid), 'id, userid, starttime, lastcheck');
     } else {
-        $presences = $DB->get_records("palestra_presence", array("palestraid"=>$palestra->id), 'userid, starttime, lastcheck');
+        $presences = $DB->get_records_select("palestra_presence", "palestraid=? and lastcheck+? < ?", [$palestra->id, $palestra->duration*60, $now], 'userid, starttime',  'id, userid, starttime, lastcheck');
     }
 
     $audiences = array();
@@ -231,7 +233,9 @@ function palestra_update_grades($palestra, $userid=0, $nullifnone=true) {
             continue;
         }
         if ($presence->starttime < ($start + $minimal)) {
-            $end = $presence->lastcheck;
+            if ($end < $presence->lastcheck) {
+                $end = $presence->lastcheck;
+            }
         } else {
             $audiences[$user] = (isset($audiences[$user])?$audiences[$user]:0) + (max($end - $start, $minimal)/60);
             $start = $presence->starttime;