Maybe there's more that is relevant to the charge than is publicly known, but this sounds like over-charging. I have read that the gun that was used was purchased a few days before the shooting. And since the boy got ahold of it without the parents' knowledge, it may not have been stored and supervised properly. But that's, IMO, negligence, not manslaughter.
IF the parents knew the boy had mental problems AND that he tended to be violent and that he was extremely angry with specific schoolmates, not properly storing and supervising the gun might, IMO, rise to the level of involuntary manslaughter, but that's some pretty big "IF"s.
Just my opinions at present, and my profession is not related to law.