I believe the main character (a male) was very weak in the beginning. He also had a female friend that stood up for him (the main character was always getting bullied). Something happens to him and he becomes dangerously strong. I think the same bullies come back after him behind the school or something. He beats them half to death and his female friend comes by. I think tells her that he has become strong now, I'm not sure exactly. The female friend freaks out a bit and runs away.
Any help on identifying this manga would be greatly appreciated ;D
________________ guitarist for band "Bucket of Sushi" =D