I't not quite apparent, but check other quests, I can't remember for sure, but there was a hurdle to progress/finish something else first (please drop better Hints, altosandherdone!). Also, you might want to check all options available to you first to formulate a plan.
Hint: You can rake in a +3/+3 P/P with the Headmaster spanking her sitting on him. Also, you have to take pictures for the humiliation and rake in the free +1 for 2 of the punishing positions each.