A friend (who should usually be in a completely different lesson, so that there is no-one missing from the class) would hide in a cupboard until the lesson was 5 minutes in, then start making loud cow mooing noises. When the teacher eventually found where he was, he should just get up and run out. Bizarre, really. This was the same person who at 15 hacked into the school computer network, and put a very basic, (4 picture), movie of someone performing oral sex on there, so that it would appear as soon as someone logged on to the PC. And he could fart at will. Genius.