You arm a teacher. You train the teacher. A gunman enters the school building.

[quakerboy]

67. One more

Police are working through the building in search of the shooter.

They come to the third door on this hallway, of course concerned this is the room where they meet a hail of bullets.

Inside, there is a teacher. A very nervous teacher, with a gun, feeling the full weight of protecting the room full of children behind them.

And now the door is slammed open and a gun muzzle pops into view

Teacher reflexively shoots. Manages to hit just the right angle to kill the first officer through the door.

From there, Best case, everyone immediately realizes what happened and stops. The teacher has to live with having killed a police officer. The kids have to live with having seen someone die, and knowing their teacher is a killer. Without getting to the Other officers or the family. More likely, there's some confusion and more people are injured on either side. Worst case.. everyone starts shooting, creating a "friendly" cross fire between teachers and cops, with kids all around, in which both sides sustain additional wounds, and even deaths.