While not school related, it’s a good example of an armed, trained citizen acting as a first responder to stop a mass attack.
From the Truth about Guns: LA CCW Stops Mass Shooting
On the afternoon of 31 January, 2016, some ten ATVs were getting gas at the intersection of Claiborne and Esplanade Avenues in New Orleans, Louisiana. Richard Macklin pulled up in a white Volvo. Police said Macklin became upset; the ATV riders were blocking the gas pumps. He pulled out a firearm and opened fire, sending more than a dozen rounds into the crowd.