You do not have to kill someone to stop them. Just be willing to put bullets in their arms or legs. The reason the police kill people is that makes their job easier and puts them at less of a risk. This is the reason we need extensive training. People have to know ahead of time what they are going to do when the time comes. They need to make their decision and they need to train accordingly.
THis is wrong on so many levels.
The reason why you do not aim for arms and legs in the midst of a combat situation (i.e. bullets are flying towards you and others around you) is because if you aim for arms and legs during the split second that you break cover to shoot back, you will most likely miss.
If you miss, there is a good chance you are now dead. Remember, the teacher will have a pistol, the assailant will have an automatic or semi automatic assault rifle. You are at a severe disadvantage
So you aim center mass because center mass is the biggest target and if you stray a few ticks away from center mass you still "hit" the target and hopefully that hit will stop the target.
Lastly, this isn't a video game. You can't "Train" teachers for something like this. There is no training that will enable you to figure out if you have the "right stuff" to jump into a combat situation and go all Rambo meets Dirty Harry...
Again, this isn't a situation in which you have backup. This is a situation in which you are outmanned and outgunned and in fact are running "into" danger against an unknown number of assailants who have superior firepower to your pistol.
This is a big deal and even a combat hardened vet would give the situation some pause because the odds are against you in a situation like that....