A particularly successful way to gently persuade your teacher onto a course of Prozac. It doesn't work if everyone does it; and for best effect it needs to either come from a couple of different directions, or from a different source each time. Advantageous in that there is no outward sign of the hummer, rendering them unpunishable. Needless to say they would cease should the teacher approach, at which point the hum would be taken up by another entrepreneurial soul on the other side of the room, head bent studiously over their long division (or whatever it was... sorry, Mr. McNally, but with that name, and at 5'2" you were asking for it. As was your car, to be fair.)

Humming was featured in a late-80's episode of Grange Hill. In the week after it aired, my school was forced to include, during the morning register, a lecture on the punishment facing anyone caught humming during lessons.

"You can hum all you want at break-time." suggested our elderly form-tutor.

Talk about missing the fucking point.

Easier, but a hell of a lot less covert, is to have the entire portion of the class not within the teachers line of sight hum the Imperial March from Star Wars as they walk. Works wonders. Or, start a round of applause when he teacher enters the room. Cease for nothing.