I can see why people would think it should be a 15 actually, I thought it was hilarious myself but certainly there's a fair amount of swearing, the sexual parts aren't really bad (or really good unfortunately ) but drugs is a bit of an issue because it does glamourise them a bit. I don't have a problem with it personally but Stephen Fry who everyone likes ends up on pills, half the girls are smoking/drinking/taking poppers, some guys get spiked with shrooms and the headmistress sparks up a joint. Also I think if they'd made it that close to being a 15, they could probably have spared losing some audience (through them not being able to go) and made it with intent of being a 15, and made it an even better film.

12a is stupid anyway though, it's a way of making adult films and still getting money off kids for it.