So say you have an armed principal. What exactly is he going to do? Take down the shooter? Schools are big, one person can be fairly easily avoided. And even if the principal did manage to confront the shooter, he would not only have to have experience with a gun, but also in confronting an armed assailant, otherwise he's just another jackass with a gun who is more liable to hurt a kid with stray fire than do any good. So for this to happen, now we have to give our principals police/military training as well as teaching them how to run a school?
Just a bad, bad fucking idea all around.