Take one science class, one Bunsen burner, a coin (preferably a largish denomination) and a pair of heat resistant tongs.
Insert coin into tongs.
Hold coin over Bunsen burner.
When hot, toss into crowded hallway jut as the student are exiting class.
Consider yourself either very clever, or a complete jerk.
Take one English class, one set of appropriate video tapes (something like Roots), one set of top-notch XXX rated pornography and the school video editing suite.
Using the master copies of the very boring tapes, insert a moment or two of high-class pornography every 10 minutes or so.
Teacher generally isn't watching said tapes as they've seen them a million times before, so you can get away with this pretty easily.
Hilarity ensues when every class from then on wants to start studying Roots.
(Pun fully intended).
Take one set of boys bathrooms and a shifting spanner.
Loosen the fittings on the urinals, just so the water will spray everywhere, but won't actually fall apart.
Wait outside the toilet block.
Every kid who went in, comes out with 'urine splash' and the piss is taken out of them for the rest of the term.
One classroom, one set of chairs and a very large supply of super glue (not what you think).
Grab said chairs and turn them over.
Apply liberal amounts of super glue to the feet.
Place said chairs back under the table and hold them firmly in place.
Enter said classroom the following morning and express disgust at the juvenile behavior of some students.
One teacher (whose sexuality may be under suspicion), one hand written note.
Said note should contain a message like:
'Hey, Sweetie! We had a great time together. Call me!' along with the telephone number of a guy at the local university.
Best result is achieved when this is constantly undertaken, your can watch, but not be seen.
One PA system, one set of 'announcements' and one ditzy bursar.
Grab one of the standard announcements (best results can be achieved with the local Christian groups announcements).
Change said announcement to contain the usual glurge, but end it with the phrase 'Be there or go to hell'.
Watch for the faces of members of said Christian group and when spotted, inform them that jokes about hell aren't funny and they will burn for eternity for uttering such blasphemy.