Why won't this work?
UPDATE acad_adv SET present = 1, teacher_complete = '$teacher_complete', comp_timestamp = '$time' WHERE acad_adv_id={$k} AND teacher_complete = NULL
...AND teacher_complete IS NULL.
Beyond that, given the information in the problem, another reason could just as well be "because this is a vBulletin forum, not an RDBMS".
