They said that some at my school too. They said every rude thing you can possibly imagine. "Get bent!" was among the somewhat milder abusive expressions, but it was still quite popular.

Another one which I never figured out was: "I've got nine! Have you got nine?" If you didn't have "nine", you were a nerd. I think it had something to do with sexual potency, but I'm still not sure.

Another one, culled from English class grammar lessons was, "How's your dangling element?", to which the obligatory response was, "Mine's straight up, man!"