public static function delete_draft_content($gradeid) {
        global $DB;
        $conditions = array('gradeid' => $gradeid, 'draft' => 1);
        $result = $DB->delete_records('assignfeedback_editpdf_annot', $conditions);
        $result = $result && $DB->delete_records('assignfeedback_editpdf_cmnt', $conditions);
        return $result;
    }