Last week with about 8 minutes left of class, I handed out scratch paper that was cut into 4ths, 1/4th to each kid, and put 3-4 quick "pop quiz" questions on the overhead (3 math, and 1 "spell my last name" since they always mess it up and it eased the tension of a "pop"). They could use their notes but not talk with anyone. We happened to be studying function composition. I told them to write down the problems, show their work, and box their answers.
They took it seriously because it was for a grade. I liked it because I could walk around and scan their answers and see who was getting it and who wasn't. After a couple of minutes anytime I saw a wrong answer, I quietly mentioned to the kid that they should check that problem. That way, later I had an easy time to grade them, the kids felt better about the "pop"ness, and I then could concentrate on the 3-4 kids that I saw just weren't getting it and help them through quickly or suggest they come in for tutoring later.