Pull the tab off of a pop can. Turn off the switch on the power bar that the teachers computer is plugged into. Unplug the computer, slip the pop can tab over the live+neutral prongs, plug it back into the still off power bar and wait.
The teacher goes to use their computer, finds it's not turning on and looks at the power bar.
Switch flips, 'BANG', scream, lights go out (breaker popped).
Na, the aluminum acts as a fuse and breaks the circuit almost immediately. It makes a bright flash and a loud sound, but it's over in a second with no real harm.
That's not to say I'm recommending this at all, it's definitely a pretty stupid prank; just that it doesn't cause nearly as much damage as you might think.
Kids have lots of stupid ideas and saftey isn't exactly the first thing they consider...
At least I never heated the handles of the tool cabinets with the blow torch in shop class. Had two separate classmates pull that 'prank'.