"Is that a private conversation or can anyone join in?" "Settle down", or "Are you chewing?" Keep saying such tedious things and you may be in for a surprise.
Bliss magazine, bible of the teenage girl, has devised a "Boredom bingo" game to liven up lessons. It prints a grid of 12 "rubbish old cliches" (others include "Would you do that at home?" and "I was young myself once") and suggests crossing each one off as the teacher says it. Three in a row merit a yell of "Bingo!" Well, at least it would prove they'd been listening.