When I was 15, I was a sensitive type kid too. My dad was the type who always was more concerned with people liking him than respecting him, so he was always sort of kissing people's asses and never really spoke his mind or stuck up for himself.
I think I was developing like that too - in HS, I was always concerned about people liking and accepting me. There were certain groups of people who did not like me and who were always sort of instigating me and it bothered me a lot. No where as severe as what you are suffering through, but I felt insecure, unsure of myself... weak, pissed off, humiliated.
I am a lot older than you now and boy do I wish I knew then what I know now. I wish I had then what I have inside of me now...
Here is what I might suggest to you. I don't think this will be easy. I don't think it will be nice. I think it will be like going through a fire, but in the end you will be forged into something different, stronger, better. A man.
What I mean is - don't lose who you are - you heart, your goodness. Lock that away for the people who deserve that part of you. Your true friends, your family.
But for the assholes around you. Change. Change everything. Your attitude, your demeanor. Make them respect you. Do whatever it takes. Shock the system. Think of your life as prison survival, because in reality what IS life? It's fuckin' prison survival in almost every regard. The strong and vulgar take what they want and the weak cower in the shadows looking for the scraps.
I won't advise you directly, but for me, I have replayed this in my head a million times. In HS there was one guy who constantly instigated me - made comments on my appearance, busted my fucking balls daily, made my life miserable while his cronies all laughed at me. As a kid I tried to laugh it off too, but in reality I was being laughed AT not with.
If I had the mindset then that I do now? I would summon all my courage, and strength and anger and balls and I would quietly walk up in the cafeteria, grab a metal food tray and when the dude made a comment to me I would walk over, not say one thing and bash his fucking skull in with that food tray until someone restrained me.
And every time after that I would do the same thing over and over. I would make sure every counselor, parent teacher - whomever knew this kid was a fucking asshole trying to bully me and I would confront him physically and violently until he left me the fuck alone. I would not care about being kicked out of school or suspended. If my life was going to be daily hell, this guy was going to have a little daily hell too.
Now I am not advising you to do this. I guess what I am saying is that life has taught me this: You can be a victim or you can be a fighter. Being a victim is safe, quiet and easy. You can hide in the shadows, avoid certain hallways or places. Being a fighter is hard. It's scary. It hurts. It's violent. It's confrontational. But when you are a fighter, in the end you will be respected. Personally I think it is more important to be respected than liked. A much different view than when I was 15.
If people know you are not afraid to bleed, you are not afraid of pain and you will do whatever you have to do to defend yourself and get the respect you deserve, they will give it to you and they will think twice about fucking with you.
The other side of this is physically making yourself into that fighter. Sounds corny, but seriously - exercise, develop your strength, your focus, your self esteem. Learn to box, learn to take a punch, learn how to throw a punch. If you can, get in front of a heavy bag and learn to use it. Visualize your opponents. Rehearse your response. Prepare yourself mentally and physically to do battle. Change your appearance. I don't mean become a psycho, I mean own your appearance and make it yours. If your haircut got fucked up, fuck it - shave your head and when someone makes a comment, tell them to go fuck themselves and if necessary, get in their face about it. You don't have to fight the whole crowd... just make sure everyone knows you are not afraid to bleed and stand up for yourself. You will probably even make a few new friends who feel the same way you do.
I never did this as a kid and I regret it - I do it now as an adult and I am confident and secure in my ability to make other people respect me. I don't look for conflicts, but I won't back down from them - ever. I box, I train and I prepare in my head for any asshole who might confront me. A drunk. A jerk in traffic. An asshole in line at the movies - doesn't matter, life will always be full of them - they are everywhere and the feed on the weak. I say it's better to be one of the strong. You don't have to be a bully or some nut who shoots up his school, but you need to be a bit of a hard ass in life to get respect and it starts with a basic shift in your thinking and your mindset.