Personally, am against arming the teachers, I think it's already absurd that schools over there have metal detectors, security guards and even their own police force, it's like prison.
But, this is logical step, it's a semi good temporary measure to counter the problem when it arises.
Law abiding citizens have stopped numberless shooters before, they have protected home owners against home inversions as well in other protected lives of the gun owner and others, the latest recent one is the guy who chased the gunman who shot up the church back in November.
Teachers come from many backgrounds, some can be ex military or police or already have trained with guns before.
The likely situation is if this happens is no different to what they do now, they will lock the school down, close the door and lock it, hide under the desks or whatever and block the door while the teacher will have their gun ready if the shooter attempts to break in.
I doubt they will go roam around the school.
That said! There is going to be many problems that arises from this.
Guns will get lost, they will get stolen. There might be the times when a teacher decides to take his own life and maybe possible others too. Anything is possible.
There is also the chance that the teacher might get shot by the Police when they arrive and run into a armed teacher, unsure who who is identify is.
There is also a fact, that the shooter could kill the teacher first and still have 30-40 kids in a classroom to gun down.
But that one teacher might be able stop him.
There is also a high chance that, they won't attack the school anymore now they understand the teachers are armed.
A lot of pro-gun people also say, that a lot of mass shootings happen in "gun free zones". I did a 2 minute google research on this and couldn't find a 100% answer on it.
Ah well, that's America for you anyway, no major firearm laws or changes will happen in our lifetimes.