It's the 2nd adult witness in the room that curbs their behaviour. It doesn't matter who that adult is, it's someone who can back up your version of incidents so they don't bother trying it on! It makes it clear to me that you are not the problem, and as they will behave when you are observed, the observers will witness your competence and be able to report back to parents that they are quite happy with how YOU conduct yourself. You now need to contact parents when you are not happy with how their children are behaving in your classes. Actually, hold off until your observations have been done but make sure that you let your HOD know about those who are being troublesome and they can decide whether home needs to be contacted.