Or hide under something.
That’s the advice from PJMedia gun columnist Jeff Sanders on dealing with an active shooter. Of course it’s easy to say what you should do. Only when confronted (by surprise!) can you know what you would do.
“MOVE!…Keep moving and you will increase your chances of survival. And if you run away, you will become a smaller target. The worst thing to do is stand still.”
Or shelter in place as the current official advice has it. Disperse. Run for your lives. The perp is likely a lousy shot and there’s nothing harder than hitting a moving target, as soldiers learn in combat.
“So… remember to do something, anything, to throw off the attacker’s plans. If dozens of people are running away in dozens of directions at the same time, he cannot focus on all of you.”
If you must shelter in place, barricade the door with something heavy. Or tie a tie or a belt around the doorknob and hold it at an angle from the door which makes entry impossible. If you must, fight back. Throw something at him, distract him. He’s very likely a coward to begin with who is not expecting a fight.