This is one from my Dad's school days.
There was a guy who had a moped of some kind and would park it out the front of the school. One day one of his friends took a soft pencil, drew a line of graphite down the outside edge of the sparkplug, and replaced it in the bike. The guy comes out after school to get on his bike and leave. now here's the scientific part: electricity will always take the easiest route, and in this case that was the graphite. This meant that the plug wouldn't spark and the bike stayed dead. A small crowd gathered, all of whom were in the know, to watch him puzzling over it for ages before finally going back inside to call a repairman. While he's gone they rub the graphite off of the plug. Repairman arrives, bike starts fine, callout fee is paid.

This school was quite an old building, made in the 20s and intended for upper-class children. It has big double doors, wide corridors, and two grassy quadrangles in the centre with glass walls so you can see inside them from everywhere. It took four strong lads to pick up the art teacher's mini, carry it through the school to one of the quads, and place it inside just before lunch. How he got it out again I don't know.

I don't know what the specific motivation was for either of these events, but the gist was that they both deserved it.
(The Ghost of Stephen Foster, Fri 14 May 2004, 17:51, Reply)