This also reminds me of two more.........
In high school, me and a buddy used to grab peoples bookbags during class, and take everything out of the bag, then turn it inside out, and shove all the books back in it, and zip it. Pretty much this left people with a big pillow thing with no straps to hold on to. For maximum effect, we would do it right at the end of class, so when the bell rang, and they looked down, it would be way too late. We did it SOOOO much in fact, that no one sitting near us trusted us anymore, so they would attempt to sit with their legs through the straps and things so we couldn't get them away. We always found a way though...
Sometimes too, we would ziptie the bookbags to the desks, so kids would end up picking up the desk, with their bookbag.
number 2 is a teacher who never taught us anything. her idea of teaching was to sit at her computer and surf the web during the whole class. This naturally irritated us, so we started going in the class before her, and disconnecting the LAN connection.
We did it everyday for two weeks... and she blamed it on kids in the class before us!!