Criminally charging the kid is just a technicality to CYA from the parents being able to sue. Parents nowadays can sue if the teacher looks crosseyed at the kid. The schoolboard has to protect itself. And what if the kid assaulted another student and the school knew the kid was a threat and did nothing? Another lawsuit?
I would imagine that criminally charging an autistic six year old will invite more trouble from the parents. There must be better ways to CYA. The incident should be put on his permanent school record. But criminal charges in this case seem like a waste of court time.