Ok...how many of these mass killings have happened at these armed schools?
um.....I believe the answer to your question is zero.
Columbine High School had armed security guards. They were trained in the event of a shooting to call 911 and wait for police response. Police response was trained to establish a perimeter and wait for SWAT. By the time SWAT entered the building nearly two hours after the shooting began, there were 13 dead, 24 injured, and Kliebold and Harris had committed suicide.
It takes more than armed guards, it takes the proper training and mindset to stop the violence as soon as possible.
Oh, and this was during the Clinton-era Assault Weapon ban that also banned "high capacity" magazines. Harris had a Hi-Point 9mm carbine (a rifle that shoots pistol-caliber ammunition) with thirteen 10 round magazines, and shot a total of 96 times.