after the Columbine Tragedy,a high school principal (and fellow USPSA shooter) and I were discussing options to arm teachers... we settled on a short barreled lever gun in .357/38spl, with a multi source dot like the Trijicon Tripower.... our reasoning was that it's operation was fairly intuitive (if you have ever watched a Western).... minimum recoil and muzzle blast... sufficient stopping power... with a relatively high hit probability of a specific target in a crowded arena
In this situation, the targets/defenders are not intermixed. The defenders will be away from the glass entry points in some what of a tight group.
If they are overrun with enough looters to cause worry over a friendly fire incident, they have lost already..
This is a defensive position and you are waiting for intruders to enter your field of fire, which stops at just inside the building, so over penetration and the longer range of a missed shot with rifle rounds could be a problem.
Handguns are out as the primary arm. You have heard it said, and may have said it, many time that a handgun is what you use to fight your way to a long gun.