That is out of a law school final, but I think Army Judge is correct. Maybe someone could say the kid was negligent, but you need to prove he had a duty to cars, breached it by throwing the ball, and it was foreseeable that an accident would happen based on the ball. Then you need to prove the school had a responsibility to make sure kids didn't throw balls in the street and it would be foreseeable etc. As Army judge says anything you need on this website is marginal legal counsel from anonymous internet posters so take it with a grain of salt.
I guess your best case would be the school knew that kids were throwing balls into the street and did nothing about it, but difficult to prove.