if only i saw this a few days ago, i have some classics, i can't claim them all, but i'll claim a few, so here we go.
if u have double-glazed windows ie: sound protection and the like, u drill a small hole, or large, doesn't matter, then fill the cavtiy with water, and then either give some goldfish a new home or just dump some pre-mixed (can't stress enough, must be premixed or it doesn't work too well) jelly, and fill the cavity again.
another good one is get four live chickens and number them 1,2,3 and 5, and see how long it is before u get annoncements like "if anyone sees a chicken around, can they please report it to a teacher"
another one is get some cows and, if your school has two stories, put them upstairs, useless u say? well, cows can go UP stairs fine, but comming down is a completely different story, they can't.
if you can get your hands on some liquid nitrogen, i know hard to get, but if u can, stick a can of shaving cream into it, and them peel the tin off and chuck the frozen cream into one of the teacher's cars, it will be fine until they open the door and it re-pressurizes and then blows apart, covering them.
another interesting one is u get a tin of paint and tip out a bit of it, chuck a block of dry ice in, get it from any theatre effects store, and then put it in the middle of the hall, close the lid and run your arse the hell out of there, once again good ol pressure will blow the lid off and the paint should go everywhere, for best effect, make it some horrid colour, fluro even.
well that's enough i think, don't want tooooooo many arrests
Phill