Reminds me of a story from Freakonomics.

A teacher would ask his students to note 50 coinflips. And based on distribution he could immediately tell which one of them actually flipped the coin.

If I would be a math/physics teacher in a high school, in a class of 30 students, I would start every class by picking random 2 or 3 students and ask them to explain the solution to me. Or do a carbon copy of one of the assignments in front of the class. Something like that.

In my high school I had a physics teacher. Every class she had a 3 rudimentary questions about the recent subjects. She would call a random name and ask first question. If you fail to answer on the spot you get an F, and another random person will get a shot at the same question. If they fail they got and F as well. Of course by the 6th or 7th person someone would manage to find the right answer. If they got the right answer they'll get the next one. If someone answered all three - they got an A. Most of the times half of the class would get an F.