I was the only teacher to enjoy a happy classroom and quality learning, with no discipline problems at all. Here are some guideline hints; they are not a list of rules but guidelines for a general attitude to adopt in the classroom.
1) Adopt a relaxed, informal posture the first time the children are likely to see you; e.g. sit on a chair that is facing backwards (but you face forwards!) or "sit" against a table, etc. i.e. avoid clasping a neat little clipboard under your arm, standing up rigidly straight to scrutinise the audience. This demonstrates clearly to your audience that you feel insecure before them.
2) Teenagers do not care as much about what you think as what their peers think of them. Do not belittle them in front of their peers, if you can avoid it.
3) Do not try to be "one of them", you are the adult and they need to know that you are secure being you. They will need you to help them to sort out their friendship, tolerance and other problems with their peers. They want to know that you will listen to them. They want to know that, whatever goes wrong one day, that you will start afresh the next. That every day is a renewed opportunity to be considered a pleasant human being again and to try again to live up to your expectations.
4) Enjoy their sense of humour, treat them like interesting, stimulating beings and NEVER allow them to sense that you feel threatened by them.
5) Give public praise for good behaviour and quiet, individual correction.
6) Use popular ringleaders as group leaders and you will reap wonderful rewards.
7) Whenever possible, base your teaching around group activities. Give clear instructions and get them on with it, removing the focus from yourself as soon as this is not strictly necessary.
8) Try to channel their energies as opposed to squashing theirs to replace theirs with yours.