One of the key things to remember is to be above their language and MO. Kill them with facts, without resorting to the temptation of stooping to their level. You'll be amazed at how influential you can be.
I know, but it just sort of slipped out. That's why I haven't agreed yet - I'm not sure if I can stop it happening - I know I shouldn't get frustrated like that, but it's hard not to. And they were getting away with calling me worse things. Once the teacher stepped in though, she banned them as well.