So in your hypothetical situation, there is an armed shooter roaming the halls, and the teacher with his gun in the teachers lounge is able to sprint to the lounge, get into his/her locker, and get back to the shooter all without bein shot him/herself? I mean, come on. This only works if the teachers classroom and the lounge are close to each other but nowhere near the shoote, and the shooter doesn't know that there is a weapon in the lounge. Under your hypothetical, the odds of the shooter knowing about the weapon in the lounge is pretty great, since one of your legs of your argument is that if the shooter know there is an armed person in the school, he's less likely to show up.
But it works on TV!