This proposal is lunacy. First and most notable point, on page 21 of an FBI report on active shooters pretty clearly says "good guy with a gun" resolved active shooters less than 4% of the time.
https://www.fbi.gov/file-repository/active-shooter-study-2000-2013-1.pdf/view
Second, consider how many school shooters end up turning the gun on themselves. That report says 40% end up killing themselves. Numerous people smarter than me have argued that in these cases, the shooting is secondary to the suicide. Should that be accurate (which I have no doubt in some cases it is) having an armed security guard makes it easier for the shooter to get the ultimate goal of suicide.
See also
https://www.fbi.gov/file-repository/as-study-quick-reference-guide-updated1.pdf
https://www.wired.com/2012/12/why-spree-killers-kill-themselves/
https://fivethirtyeight.com/features/suicide-prevention-could-prevent-mass-shootings/
Best case scenario? Shooter is stopped. Worst case scenario? New target, shooter can retrieve additional weapon and ammunition, response is delayed because shooter uses radio to call in an all clear, and other responses are delayed because on site police 'have it covered.'
Utter lunacy.

