I'm sorry to hear about your son. I hope that he is feeling better.
As far as the school being responsible, that is a legal question and one that you probably want to seek council on. Each state varies. Just keep in mind that the school board has money to fight lawsuits that they feel they can win. A question you might want to consider is, could the school have prevented the incident from happening? What I mean by this is, were there supposed to be teachers present outside the school build at the time the attack occurred? Was school in session or did it happen after school hours? If this happened at dismissal and teachers were supposed to present where were they? Did they see anything? If supervision should have been in place, then you might have a case.