Flamethrower
Turning on the gas in chemistry and putting a lighter to it. Obvious really. Best time was when Colin Stone lost his carefully combed and hairsprayed, nu-romantic fringe in a puff of foul-smelling smoke.
written by Di*in* B*stard, approved by Susan

In my experience, the best time to do this is when you are doing a demonstration for the primary school kids, and 'accidentally' turn the wrong tap on, very neatly setting fire to their parents.
written by excluded pupil, approved by Log

Taking a length of flexible tubing and spraying a can of lynx into one end, while holding a cigarette lighter to the other will make a rudimentary playground flamethrower.

Unfortunately the fire has a tendency to burst out of both ends, injuring both victim and assailant.
written by Co*or Fr*nklin, approved by Susan

Can also be jerry-built from a cigarette lighter and a can of Lynx. This had the added bonus of being portable and made everything stink of "Java" or "Africa", depending on whose sports bag you raided.

(If there is a more powerful emetic than the stink of burnt teenage hair and fucking Lynx Java mixed together, I haven’t found it. –Susan)
written by Dr*Ringp*ece, approved by Susan

Of course, attaching those orange bunsen burner tubes to the gas taps made directional flamethrowers... very good for fights. This does, however, destroy the tubes after a while, so make sure you steal plenty of them.
Filling each others pencil cases with gas was also fun, although I forget exactly why.
written by Th* Bit*ern, approved by Log

Attatching a bunsen burner to the water tap rather than the suggested gas main provides a suprisingly high powered water pistol. Closing the opening at the bottom of the burner is advisable.
written by excluded pupil, approved by Conor

A surprisingly inventive variation on this was practiced - once - by a clever fellow at my school. In a quiet moment when the teacher wasn't paying attention, he connected the gas tap to a length of bunsen hose, and the other end of a water hose. Then he turned them on to see which would win.

The result was a clear victory for water pressure, a victory which proved to be irreversible as the underground gas submain filed with water. The gas pressure wasn't sufficient to drive it out and it had to be dug up and drained at a cost of thousands of pounds.
written by Ja*k R*rebit, approved by Mansh