Physics
My A-Level physics teacher was ace. Particularly memorable was the *experiment* he once set up: a large tin can, with holes in the bottom and top of it. Fill it with gas, and light.

It burns very gently for a few minutes. Then, the concentrations of gas and oxygen get to the right level... And *BOOM*. Large, very loud explosion, and projectile tin can.

Terrifying at the best of times, but even more so when set up at the start of the lesson before you arrive, and teaching commences as normal... For a few minutes; before said explosion, and several members of the class falling from their chairs in absolute terror.
(Merrep, Fri 30 Jan 2009, 19:52, 1 reply)

ditto
our chemistry teacher used to do this to get peoples attention when the class was misbehaving. Other ploys included burning powdered sulphur to make people feel sick. a dangerously insane human being
(Joe Guevara is well known for stealing your biscuits, Sat 31 Jan 2009, 19:19, closed)