This is an excellent real world combat situation, that could have been deadly if the guy had not had the courage to react and attack. Those of you with the responsibility of teaching others life saving techniques should have your students watch and learn.
I will never crictize anyone for taking action to save lives. I do not know if the guy had any previous training, but I would like to suggest some areas of improvement:
Someone brings a gun to a campus with the intent to kill is an enemy. In the future, never leave that enemy with any ability to attack again. You will notice after he took the gun away, he ran into another room, allowing the attacker to access a second weapon. You had him down on the floor, at least stomp the shit out of face and break his combat mindset. Break his will.
You will see him come back again, after the attacker pulls a pistol, grab the attacker, and hold him in a "bear hug". true courage.
To save yourself from violence, you must be willing to deploy violence. Please do not misunderstand, I am not crticizing the young man, just pointing out what (IMHO) other avenues could have been taken.