I think there was a scene with one of the good guy's bycicle, where this bad guy stole or broke it... can't remember...
This really blonde guy is about 14/15 years old (just like the good guys). He always attacked the good guys for no-reason. He was very violent because (and if I rememember correctly), his own father hit him. In one particular occasion, somehow he got one of the good guys (a fat boy) naked in the middle of the school's corridor, being a terribly embarrasing moment for him, since all the kids from the school were watching. He was lying on the floor, crying, while one of his friends went to help him and told everyone not to watch or laugh.
At the end of the movie, these good guys get revenge by tying this bad boy and treating him with a gun (that, if I am correct, belonged to him). One of these good guys was so pissed off that wanted to shoot him, but another guy tells him that it's not worth it.
This blonde kid had a punk attitude and I remember him wearing a black leather coat. His face was very similar to Macaulay Culkin's.
This is all I can remember now. I hope this helps.