calculate the Sum of note pupils in class

Question

I have two model pupils(..., note, class_id) and class . I want to search the sum of the scores of pupils in each class. I tried that but not worked :

$this->Class->Puplis->find('first', array(
                     'fields' => array('sum(Puplis.note) AS total'),
                     'conditions' => array('Puplis.class_id' => $id),
               ));