When your survival instincts kick in, the choices you make are critical. While it may feel natural to hide yourself away somewhere and hope it passes you by, acting positively and taking the initiative will increase your chances of getting out alive.
Of course, all of this would also be totally unnecessary if everyone was able to freely carry. No one would bother trying to shoot up a school if they knew at least 1/2 of the people there could and would shoot back.
And have you seen most teachers? Yeah, they're going to go hand to hand with a shooter and take them out. Sure. Just another example of GRRRRLLL power trying to be realistic.