IF this is as you have said, the kid was there to pick up other kids.. AND it was after school, that rule does not apply. He was not at the school he attends, and was visiting OR on campus for the purpose to provide transportation to other students.
And if it was after school, then it is a personal attack/assault. You need to step this up.
If you can't bill the kid or parents, then go to civil court to recoup your bills. He now has the assault on record, AND you can charge the school district with neglect, and failure to provide immediate medical care for your son, and failure to follow proper procedures in contacting you, contacting the police. And you can name them as co-defendants in the suite so you can have your bills paid.
Just a thought
Carol