1. Mass murderers who plan on killing themselves after the spree are not deterred by posthumous legal penalties. Really. Trying to disarm everyone who can theoretically snap is impossible — witness the Russian cops doing mass shootings against the disarmed population.
2. Mandatory lock-downs deny escape to those whose only hope is being a swift and difficult target. Interior doors usually have glass sections and do little to stop armed intruders.
3. At the colleges where I taught in the past decade, about 25% of the teachers and engineering school students carried pistols, up to 10% of the graphic design students. When VT shooting happened, a co-worker said “Thank god our students carry! If this crap happens here, we can hide behind them.” Granted, I wasn’t teaching at a K-12, but the same reasoning applies to them. In a typical school with a hundred or more staff and teachers, it’s likely that a half-dozen armed first responders are on site, and at least one of them would be close enough to fight back.