The teacher would not be expected to hunt down and neutralize the threat, only to either return fire or slow the attacker.
Meaning that if the teacher has a loaded gun already in hand when a shooter entered her classroom, she might stand a chance of shooting him first. The first thing the teacher is supposed to do is lock the classroom door. If the shooter enters before she is notified to lock the door — surprise! — would she be able to shoot him first? Seems doubtful. So that leaves the teacher confronting the shooter when she happens to be elsewhere in the school. Do these teachers carry a loaded gun on their persons everywhere? Maybe they do. I honestly don’t know.
Again, I am all for teachers being able to protect themselves and their kids. I am skeptical whether this would prevent all deaths caused by school shooters. It might deter some shooters and save some lives.
You would still need police to hunt down and neutralize the shooter.