Please. I'm hardcore pro-2nd Amendment. But that doesn't wash here.
This guy walked into a one-room school building armed with two handguns and a shotgun. Facing pregnant women or women carrying infants. If the teacher would have had a gun and gone for it, it would have been futile and could have increased the death toll if the perp simply started firing at that point.
Sometimes there is stark evil in the world that simply will exert its will over our existence. No what-ifs that you or I could ever conjure up could ever counter it.
This is one of those situations. All we can do is pray such evil crawls back into the hole from which it came.
He picked a school in a community of non-violent pacifists.
He could be certain that nobody around was armed, had any martial arts expertise, had any military training, nor the mindset of dealing with sudden savage violence. He picked his target carefully, in that he wanted to be absolutely certain of no resistance.