Why the school did their job. They had no way of knowing the kid had an illegal fire arm on him.
Even if the parents do somehow not go to jail for this they will be sued into their next life.
I would guess that the argument against the school is going to go this way. You have at least two teachers who are concerned enough about the threat this kid posed to alert the school (in hindsight, an excellent assessment by those teachers), and a school that is at least concerned enough about the threat that it calls in the kid's parents (in hindsight, another excellent assessment).
The problem, though, is this. If you are a school, you can't just leave it up to the parents to handle. You have to make your own independent assessment of the risk to the rest of your students and the staff (in hindsight, not that great of an assessment).
That, coupled with the school having the deep pockets in this group, is why I think they'll be targeted. Not saying it's necessarily right that the school be held accountable. I don't know precisely what they did or didn't know. But they are certainly going to be a target for the victim's lawyers.