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.)

One of our physics teachers was called Humphrey Payne, and for some reason, his Christian name became a source of enormous amusement for the whole school. In one physics lesson, determined to irritate Humphrey, Sean Reuben put his hand up and said "Can I tell a joke sir?" and then proceeded to tell a joke which began with the feed line: "What do you call a camel with three humps?". Payne, incensed, turned a bright shade of red and ordered Sean to get out of the class. Sean feigned innocence beautifully, looking as if butter wouldn't melt in his mouth so Payne chased him madly out of the room. Once Sean was outside, Humphrey slammed the door so hard that the security glass in the door shattered. The rest of the class at this point sat in shocked silence.
We erupted into uncontrollable laughter when Sean popped his round the door a few seconds later and with a look of angelic innocence and supposed helpfulness on his face, pointed out to Humphrey, "Sir, I think the glass in the door just broke."

More specifically, hymn books placed upright on the chair in front, in line with the arse crack, just before the kid sits down at the end of the song in assembly. Many a comically hurt look when they turned round.

While shepherds washed their cocks by night,
While watching BBC
The angel of the Lord came down
And switched to ITV

Like cellar door, jism or flange: one of those words that rolls so sweetly off the tongue that you say it again and again and again, up until the moment when you introduce yourself as "Mr David Hysterectomy" to the drama teacher, and she runs out of the room.
Girls cry at anything!