How about having an armed guard at the school?
Columbine had a single armed doughnut muncher, overwhelmed and outgunned by Eric Harris and Dylan Klebold, he ran away *to call for backup* once he realized he was up against armed opponents.
The real answer lies in utilizing a force that has no problem in dealing with multiple hostiles, and whose personnel would be much closer in age to the kids in the priso educational institutions themselves, and not viewed as the enemy.
This came up in the 1920s, when a series of brutal armed robberies of Post Offices was accompanied with the murders of the postap personnel witnesses. President Coolidge sent in the Marines to gard the mail, the facilities and the employees, and that was the end of that.
Semper Fi
We don’t have enough Marines to guard half the schools. Total personnel in all the Services is less than 1% of the population and 90% of those are not in combat skill areas.