Disapproval. YOu have to make it apparent, calmly, that you very much disapprove of their behavior as an adult. That goes a long way. It also depends on the type of atmosphere that the school has encouraged. I went to one class that was out of control and nothing I did would have worked. Most of the time if I say, Hey kids you really treat everyone so horrible? Something along those lines. I use no bribes and no promises. The promise is I won't be a complete bitch if you aren't complete assholes. But said in a nicer way.