I think John Oliver did a deep dive and found in the history of shootings, a small handful were stopped by "a good guy with a gun" while a significant number beyond that were stopped by people without guns.

He went further: the good guy with a gun often were used for disciplinary measures (read: send black students to the school resource officer) instead of guarding the place. Not only that, but in school shootings were more violent where there was good guy with a gun.