Here's how to respond if a shooting breaks out.
There's no such thing as a perfect world in which guns and bullets don't exist. Would you know what to do in the event of an unexpected shooting situation?
The first thing you should do in a shooting crisis is to remain calm, even though your head might be telling you to fight or escape. However, an attempt at fighting or escaping may not always be possible, regardless, maintain calm to determine what is possible.
Be aware of your surroundings: people nearby, what they're doing, odd behaviors, unfolding situations. If you spot dissonance (e.g., an escalating argument), look for possible exits or safe spots to get to, rather than get closer to the unfolding train wreck. If someone brandishes a gun, you'll then immediately know where the fastest exit or barricade is. Always know where the exits are in any room you're in.
In general, if you're in a threatening situation, especially if a shooting occurs, 1) run, 2) hide, and 3) fight. And not always in that order.
If you see the exit, run. If you can see the gunman, he can see you; drop any belongings, crouch and bolt away to a safe place, even if you become injured. Then call 9-1-1.
If there's a quiet, dark room that you can lock yourself into, do so. Bring other people with you if possible, but keep them quiet: no screaming. If the door doesn't lock, block it with furniture. Stay put until the authorities find you.
If running and hiding aren't options, you must fight: a very last resort, however. If possible, recruit others to join forces. Use any weapons available (chair, lamp) and fight for your life. If the gunman's weapon freezes, lunge at him or sprint away. A 120-pound woman can do this; 120 pounds is a lot of weight coming at a full grown man, whether it's a big rock or a woman.
This boils down to situation awareness and preparedness. Never think that preparing ahead of time for a shooting that probably will never happen is a form of paranoia. It's simply being proactive.