if you think it was because he was only carrying a handgun, you are showing your ignorance. I can assure you that anyone that has shot a 9mm or similar with regularity and a pocket full of clips can neutralize a lot of people quickly.
BTW, I don't intentionally leave out pertinent information to cast a preferred light on a conversation. I also have not found any articles that stated the SRO was nearby... only that he pursued quickly. Regardless, if you allow able bodied willing staff to carry a gun, there would always be someone close by.