Ahem. Columbine had an armed security guard. He exchanged gunfire with the killers. When Sheriff’s deputies arrived, more gunfire was exchanged – and the killing continued. Eventually a SWAT team arrived – and the killing continued.
The six deputies were on the scene within five minutes, the SWAT team within seven minutes. On many high school campuses, it could take a security guard minutes, not seconds, to reach (to locate?) a problem area.
The excruciating details of the Columbine response are here.