There is quite little information on the situation as a whole, so judging who is at fault in this case, is very difficult.
I believe that both are at fault, but I am more curious about the victim, and here is why: first of all, as we know from the text, no one wanted to stand up for the victim in the first place. “Other students called him names, make fun of him, push him around occasionally, cheat him out of his possessions etc..No one is prepared to defend or support him against his abuse” so we can assume that this was going on for quite some time now. His decision to payback the bully(s) was a forced measure, and therefore, was not a random unthought action.
I believe that the victim’s family should pay for the medical care, but the bully should carry out consequences like being suspended from school for a while.