Get into their classroom after they are gone for the day, and turn every desk and chair upside down and facing a different way....
Unplug a part of their computer that they use all the time so it wont work when they try to use it.
I LOVE the foil and/or the saran wrap idea......
Take any poster/picture on the wall down, and put it back up, but crooked...
YAY!!! This sounds fun!!! 4951