If you can avoid the fight, then just avoid it. The sort of people who go round looking for trouble really arent worth the effort and the trouble you could get into as a consequence of the encounter will probably make you wish you had just walked away.
Of course sometimes you just cant avoid conflict and to be honest theres no feeling like giving a bully a taste of his own medicine and seeing the look on his face when he realises someone has actually stood up to him (or the look of satisfaction on his victims faces after youve planted a good un on him!)
In this situation some form of training will definitely help. Most fights I saw at school involved a quick flurry of punches and some nasty stuff on the ground before it got broken up by teachers or other pupils.
Therefore learning both boxing and a ground/grapple based martial art will definitely prepare you as well as giving you a focus and direction for your fitness training in general. Personally I learnt how to handle myself on the rugby field (plenty of punch-ups in between the rucks and mauls) but I dont think that would be an option for you.
If push does come to shove, and you realise that you cant get out of a dodgy situation, then for your own safety get in there fast and hard and finish it before it really starts.
Forget the stances and guards, just floor em. A mate of mine is a pretty lethal street fighter, partly because he’s frickin huge and fast, but mainly because if trouble starts he’s already landed 3 or 4 punches before the other guy has finished shooting his mouth off and posturing.
However, I would realy urge you to always try and avoid trouble though if you can. I saw a great tragedy when I was your age.
A friend of mine got into a fight in the school yard with a slightly older kid for no big reason, just playground politics. My mate came off pretty bad and had his head smashed into the concrete a few times before it got broken up. The next day the whole school was called into an assembly to be told that he had been taken into intensive care with a blood clot on his brain amongst other injuries. He ended up with the left side of his body semi-paralysed and a significant reduction in his mental capacity. He was never the same again.
Hope that helps a bit, its just what Ive learnt from my experiences in a school where fights were commonplace. Stay safe man.