public function add_message($message, $class="notifyproblem", $align='center') {
        global $SESSION;

        if (empty($SESSION->lesson_messages) || !is_array($SESSION->lesson_messages)) {
            $SESSION->lesson_messages = array();
            $SESSION->lesson_messages[$this->properties->id] = array();
        } else if (!array_key_exists($this->properties->id, $SESSION->lesson_messages)) {
            $SESSION->lesson_messages[$this->properties->id] = array();
        }

        $SESSION->lesson_messages[$this->properties->id][] = array($message, $class, $align);

        return true;
    }