At Columbine, there was a deputy sheriff in his car outside the school. It took him a while to respond, but when he did he traded fire with one of the killers. They exchanged about ten shots. I am thinking of a gun in the main office, with someone in that office able to use it. He/she would be a lot closer to the action than that deputy. Think of the analogy of a fire extinguisher.
What if that someone is in a meeting or called out of the office. In order for a firearm to be useful in a time of need, it must be immediately available.