I think you've blown the whole situation out of proportion...it sounds like you haven't given them any significant reason to give you trouble...but you're ACTUALLY scared that they're going to KILL you? They're NOT going to do that. Worse case scenario, you end up in a brawl with someone. They probably won't start any trouble on school grounds, so you don't have to worry about punity. Fight back, but know when to quit. There's no sense hurting yourself worse then you need to, and there probably won't even be any bloodshed, just a couple bruises. Nobody need fear bruises.
If they DO start the fight at school, the punishment is probably in your favor anyway. It sounds like you're not a troublemaker in school. Based on the fact that you play a lot of instruments, my guess is the school faculty will recognize that you weren't the one who started the fight, and they'll go easier on you. At that point, you probably end up with a detention. No big deal.
If you do end up getting into a fight with one of these kids, my guess is the others will back off (even if you get your ass kicked). It will have already been done, there will be no novelty in it, and they will no longer find any sense of self-gratification in fighting you if someone else has already beaten them to punch (pun intended).
In the future, work out and bulk up. Sign up for martial arts. Learn to defend yourself. It probably won't stop people from being assholes to you, but if they start to get physical and end up getting floored, at least they'll learn a lesson. Plus it will give you more confidence.
Ultimately, it's just part of growing up. Has anyone NOT been in a fist fight? I was only in 5 or so during my years in school, and I survived them all with no blood to show for it. It's nothing that will leave you permanently scathed.
