Hmm I have a few ideas
Theres always the penny trick. Wedge a penny into the side crack of the door of every classroom so you cant open them
Flour! Get one of those heavy duty blowers, pour flower at the crack under the door and make it snow!!
My personal fav, which I did all througout high school is the water fountain. There are 2 holes, one big and one small. Plug up the large one so all the pressure comes out of the small one...you SOAK people and its the most hysterical thing ever
