For John, BLUF: Not every situation calls for massive retaliation. Sometimes a person has lost control and confrontation with a strong personality will create new "facts on the ground".
Here is an interesting take on confronting a gunman on a school yard. It was a "Charter School", but it was South LA. Per The Daily Beast, School Principal Shanley Rhodes confronted a young man pistol whipping one of her students. The man with the gun knocked Principal Rhodes to the ground, but she got up and faced him again, and then he ran away. I don't think the young man with the gun was intent on mass murder, but he did have a gun.
The aftermath of the confrontation is even more interesting than the confrontation itself.
Regards — Cliff