Have you ever thought about what you would do if you were in a public setting when a mass shooting starts? If you haven't, you need to start thinking about it right now!
The first and most important thing for you to do is to be mentally prepared that something unthinkable could happen to you. If you can't imagine this happening to you, it will be hard to take any action when it does.
Your plan should be the following (notice none of these involve doing nothing):
1. Get out - your first response should be to get safely away from the situation. Even if you are carrying a concealed handgun or other weapon - that weapon is for the protection of yourself and your loved ones.
2. Hide out - if you are unable to escape, look for ways to hide or develop barricades against the attacker.
3. Take out - if you have no other options, you should act as violently as possible to stop the shooter. If you have a handgun, this is the time to use it. If not, improvise a weapon or just counter-attack with your fists.
Check out the excellent active shooter video from Liberty University in Lynchburg, Virginia. Concealed carry on campus?!? That is a school I want my kids to attend!