He didn't just threaten to bring death to the school he posted images of the gun he was going to use and more.
The school was sure as hell notified and they blew it off. Their the ones that should have turned it over to law enforcement.
School administration's like to act like judge/jury/executioners, they rule their kingdoms like they are God's.
Most all the School Admins I have known over the last 30+ years worry about the welfare of their students and also worry a great deal about liability. Plus they themselves are answerable to their own superervisors at the district level.
If that kid’s posts were noticed by someone, identifiable as his, and as specific as you state AND reported to the school then it looks like the school dropped the ball. Doesn’t seem likely to me that was the case.
Time will tell.
"...if the gentlemen of Virginia shall send us a dozen of their sons, we would take great care in their education, instruct them in all we know, and make men of them." Canasatego 1744
