They did so in direct response to terrorist attacks on Israeli schools. Since the change the terrorism has stopped.
No school shooting in this country has EVER been stopped by an ARMED TEACHER. One was delayed by the Principal retrieving his own gun from his car in time to keep the kid from fleeing and continuing the spree, but the damage at that school had already been done.
At my inner city high school there are two other NRA certified Firearms Instructors (besides me) and a half dozen other former military folks (besides me) of that number about half are former SpecOps folks (I was both infantry and an armor officer). I would allow any of these folks to carry concealed any day of the week..but it would be critical that nobody at the school know of the weapon being present. Because there are so many known and dangerous felons at the school, that train themselves to disarm and murder police officers, they would not hesitate to set up a discrete ambush to relieve anybody of their weapon.
These punks have no fear of anything. They don't fear authority, they don't fear the law and they don't fear death. That's a very dangerous combination to have present in school. But this is the enduring situation in all schools...even in your town.
How could we? We don't allow teachers to be armed in our schools. However, a better question remains:
How many school shootings could have been prevented or minimized by allowing teachers to be armed?
And I certainly agree that it's critical the thugs not know who's carrying.