There is negligence from the side of school authority. It is an accident or happened at the time of game period. There is no willful crime from the side of your sons class mate .Don’t think it be a big legal issue in life. Most probably the school authority not known about the clavicle fracture and injury on knee .Think if it is happened due to the push of your son to other child what will be the reaction from your side.
But if you think to file a case or complaint against school authority,then move for ward but has no merit at all,but surely you must intimate about the clavicle fracture and injury happened to your son at the game time to school authority.