Why? Unless the gun can be proven to have been purchased or stored illegally, there's little to go after them for. Maybe they're "bad parents," but that's not illegal. Maybe the kid is just bonkers crazy, beyond hope, and they've been doing everything they can to control him. We don't know enough of the facts here when it comes to the parents.
To me, it looks like they have their own problems dealing with this nutjob kid. I remember reading that one of them is usually with him at school but were not that day. The kid is obviously deranged/mentally ill. I'm sure the parents have had enough issues dealing with that over the last few years.
Looks like the school and the assistant principal are the ones who need to be held accountable.