When bullets fly there are three things you can do.  First, move.  Second, seek cover.  Third, shoot back (if you are armed).  In this video, Glen Evans, co-founder of Church Security Alliance discusses the proper use of cover, avoiding concealment (unless that's all you got), and tactics to shoot back or advance if there is no cover to be had.

Also, listen to an interview (link below this video) I did with a pastor that implemented a security plan from scratch and how they continually improve the program through training the volunteers.