Ya, I have heard it all before.
Its fine you pack everywhere you go, good for you. I have no problem with that either.
But you are at WORK during the day, you are not standing in your kids classroom standing guard. You wouldn't be there.
...and if they have time to barricade themseslves in the classroom, they have time to run like hell out of there!
As a side issue on school security, thats another debate, and perhaps its high time to see an increase in armed security at schools.
I also have no problem with a teacher who say has a CCW with a gun in class, so I will elaborate my point:
If you were on a mass killing spree, and even if you were in a school where you knew a teacher might be armed...you are just going to know who to shoot 1st. After that, its all fish in a barrell, and you know it. So, the only sensible thing to do, is to have a policy of immediate evacuation, coupled with aggressive police response, and better school security methods, including armed guards, who hopefully would be quality people, not 5 dollar an h.r guys that would not risk themselves when called upon.
In any case the question was how to defend in the case of a mass shooting, and there are many states, and countries (other than the USA) where teachers and students cannot have CCW, so its not a serious "fix" in many places, and questionable even in places where gun laws allow CCW.