Haha, soooooo many stories!!!
We used to have a double decker, first couple of years, if you were hard/cool you could sit at the back downstairs and last few years upstairs... sitting in the corner at the back downstairs was always potentially dangerous as we'd have random bundles squashing whoever was at the end 
Used to have regular food fights, that would get silly out of hand over time, until people were bringing in yogurts, eggs etc just for the food fight... was generally at least one proper fight a week, I've seen one of the upstairs windows pushed out where the lad was thrown against it so hard, and a kid get chucked down the stairs, got so out of hand we used to have a seperate member of staff on board to try and control everyone, but they were generally too scared the whole bus would turn on them, so was pointless really.
Often had the bus turned round and taken back to school, main one I remember was kids throwing batteries out the windows at other cars!! That and general vandalism of the bus, seats ripped out... thinking back it was just a bus full of animals really, but you didn't think anything of it at the time, as everyday was like that...
Haha, and we didn't just smoke cigs at the back up the top, my mate used to bring his bong in everyday... and everyday we'd end up having to use orange squash, cos no one ever brought a bottle of water in 
Funny memories, but I'd be ashamed if I could see myself now if I'm honest