Examples why the "hero-with-a-gun"-fantasy doesn't work:

[DetlefK]

http://www.dailykos.com/story/2015/10/03/1427420/-Armed-Vet-proves-NRA-wrong-and-explains-why-he-didn-t-confront-Oregon-Killer-with-Good-Guy-Gun?showAll=yes

Turns out, there were multiple good-guys-with-a-gun on the scene when the mass-shooting at Umpqua Community College happened. They just decided to not join the fray.

Why?

“Luckily we made the choice not to get involved,” he explained. “We were quite a distance away from the building where this was happening. And we could have opened ourselves up to be potential targets ourselves, and not knowing where SWAT was, their response time, they wouldn’t know who we were. And if we had our guns ready to shoot, they could think that we were bad guys.”

Or other good-guys-with-a-gun might have mistaken him for a shooter.

Or some innocent person might accidently get shot:
Norman Williamson carried a concealed gun for self-protection. One evening, his group of friends got into an argument with a group of strangers. One of the strangers pulled out a gun and opened fire. Norman Williamson returned fire in self-defense and accidently killed a bystander. He got a sentence of 25 years, despite shooting back in self-defense only after the attacker had already opened fire.

Or the mass-shooter might kill the concealed-carrier and gain an additional gun and ammo:
When Jared and Amanda Miller went on a shooting-rampage in Las Vegas, gun-carrying Joseph Wilcox tried to stop them and was killed.

Yes, there are stories of people using household firearms to defend themselves from intruders. In that circumstance, the homeowner has several advantages: They know the building and room layout, even in the dark, better than any intruder could, for example. It's not that concealed carry never works. But on a school campus with hundreds of people, any of whom may be a potential victim or a potential shooter, adding additional armed persons of unknown intent into that situation is again is not optimal.