We "vaselined" every door knob in the school, and we released several live chickens, a sheep, a pig and a couple of goats in the school cafeteria. (to this day, I don't know how/where we got the farm animals.
This is a great prank, the results aren't immediate, but, they are awesome. If you can get into the principal's office (or a teacher's lounge)...first you have to buy some frozen shrimp or fish, then get into the office/lounge, either unscrew the air vent cover(s) OR if they have those chairs with the pneuamtic lever (to raise or lower the chair), take the seat off of the stand. Then put the frozen shrimp/fish in the vent or in the stand, and replace them.
In a day or two, the office/lounge will smell like rotting fish, and noone will know where the smell is coming from.
Have fun.