I would jam the door with one of the chairs (lean chair back, place chair so the top of the back makes contact with the bottom of the door knob/handle). I would then use the tables/desks to create a barricade in the back corner to catch the attacker's attention if they manage to get in. I myself would place myself against the wall next to the door. If the attacker managed to get in, I would attempt to disarm him, breaking his trigger finger in the process, and either hold the attacker up at gun point by their own gun, or beat them unconscious with it.