To all the wonderful and dedicated Assistant Principals out there: please note that it's the teacher who is the target of the joke here, and not the hard-working ensurer of quality instruction for the entire department. Good job, you!
EDIT: I was relating this joke to a coworker, who immediately made a comment about "dark sarcasm in the classroom".
Dang! Should've seen that one. If nothing else, that should have been the 'alt' text.