Mass shootings are incredibly rare, if caught in one, they are an incredibly dangerous scenario. While many TFB users may be tempted to use their concealed firearm, the most obvious course of action to survive is NOT to engage the active shooter (they can shoot back).
Surprisingly, the City of Houston (go Texas!) put together a solid and high-production PSA on surviving an active shooter. They show the steps in detail and provide tips in common language, demonstrated action, and appropriate candor.
Kudos to Houston.