Running, if you have a clear exit, is the best choice. Don’t worry about any of your stuff, just drop it and leave it behind. Try to get other people to flee with you, if they will do so immediately. If you can see a shooter, he can see you, so try to crouch and run as fast as you can so that you are not an easy target. Keep running until you reach a place of safety. THEN call 911. if you are wounded, KEEP RUNNING as long as you can.