You do what your training has taught you. I’m responsible for my family and myself. My decisions will be based on that. You first prepare for the possibility by knowing your surroundings, where exits are located, where entrances are located, if you hear something that sounds like gunfire then react as if it is rather than kid yourself. Get your family and yourself out of the building as fast as possible. If not possible to get out of the building then find as secure a spot as possible to hide, one that can also be defended with the door directly in front of your spot. Desks and tables likely won’t stop rifle fire or larger bore pistol fire so prepare as many layers of defense as is practical between you and the door. If someone with a firearm comes through that door then don’t waste time trying to talk them out of harming you. Shoot to kill with as many rounds as necessary to neutralize the attacker. When the police arrive let them know where you are, place your firearm on the ground, show the police your hands above your head facing them and follow their instructions precisely. G
When you call 911, tell them what the good guy (you) is wearing and where you are standing. The only change I would make is that I would also tell them ahead of time that when I hear the sirens, or see the units headed my way, I will lay the gun down and put my hands over my head.