
Ohhh they're brilliant! Real classics! I'd love to meet your friend!
I've done the humming thing myself. Another good one is (yes, I got this out of a book but it was hilarious!) every time the teacher turned to write on the board the whole class shifted their desks back a couple of inches, ever so quietly. By the time the bell went, the whole class was squished up the back wall. The teacher went beserk and it took us 10 minutes to get out of the classroom but it was worth it....
Oooo another one my mate did was draw red lines all down the outside of her hand in drippy ink, cup her hand and pretended to have a nose bleed. She got let out of class straight away!