Good to see that some people on here are still mature. I got bullied at school and it was a problem for me, although not as physically severe it caused me to change school, and all turned out well.
I know you can't run away from all your problems (I still get abuse from those guys and got pushed out in front of a car by one) you can still try to have as little contact with these people as possible. If you were to talk to this girl i would advise an e-mail, letter or text, means you have black and white evidence of your words so she can't twist them or neither can other people.
I wouldn't recommend changing school straight away, but if it becomes something that keeps you awake/makes you feel scared of going into school it could be a good last resort, it was for me.
Believe me, it will pass over, not in the space of 2 or 3 years, but do your best to weather the storm and you will have gained far more knowledge, maturity, empathy and understanding than those guys would have gained hurting you, and also there is a sense of real pride that is second to none.
I mean 15 is hard enough age, not old enough to be recognized as an intellectual peer of adults, but not young enough to get away with stuff, I can assure you i went through the same stuff that you did (maybe not the same severity of injuries) but really physical injuries will last 10 months max, and the mental anguish these guys will have after (Even if they are utter retards) will stay with them and haunt them for a lot longer
Good luck, right behind you,
shefellover93