Ohmmeter
A setup for a mischievous physics teacher's prank.

[Teacher contrives for the class to revise electrical circuit symbols]
Teach [draws a circle with a 'V' inside]: "What does this represent?"
Pupil A: "A voltmeter, sir."
Teach [draws a circle with an 'A' inside]: "That's right. Now what's this?"
Pupil B: "An ammeter."
Teach [draws a circle with an 'O' inside]: "Well done. How about this?"
Rather too keen pupil C [adopting the air of having cracked the tricky follow-on question]: "Is it an ohmmeter, sir?"
Teach: "No. It's a Mexican riding a bicycle."

A year later, Pupil C, while remaining frustratingly un-Mexican, was knocked off his bicycle by a passing car.
written by an*nymou* user, approved by Rosy

Our maths teacher was of much the same bent, although he added a rather sinister twist:

After doing the usual inoffensive "Mexican on a bicycle", "Mexican on a bicycle going up a mountain" and so forth, he then drew a circle with three pointy triangles inside it, radiating from the inner circumference.
"What's that?" he asked.
After a few guesses, we relented.
"What is it then, sir?"
The maths teacher looked pleased with himself, and proclaimed:
"The last thing a black man sees after the Ku Klux Klan have thrown him down a well!"

Silence. Utter silence.
written by an*ny*ous *ser, approved by Jamie