While some of the critique, offered by second amendment supporters, is spot on, I think your central thesis has merit. Many gun owners & advocates carry on as though life were a movie, where the filmmaker shows you exactly what you need to know to successfully resolve the situation.
Take the Aurora Theater shooting, for example. Many pointed the irony of the theater having been a "gun free" zone. If only some or all of the attendees had been armed, right? But, sitting there in the dark, you attention held by the screen, what do you do once the shooting starts? Who do you shoot? Some other guy or woman with a gun? If so, you're going to hurt or kill an innocent.
Same with active shooter scenarios. You're certainly not going to see the situation initiate. Do you go after anyone you prowling the halls with a gun?
That said, I can't entirely blame the principal, not after Sandy Hook, where all teachers & staff could do was put their bodies in the way. Obviously having a firearm on your person, or readily available, complicates your life & potentially that of people around you, so the potential payoff had best be worthwhile, & that is difficult to assume.