Whether it's at a school, in a workplace or at a movie theater, an active gunman poses not only a serious threat, but also one that's largely unpredictable. Just like a roving bear or aggressive dog, a gunman's attention is likely to be drawn to moving objects. It's also impossible to outrun a bullet.
The FBI tells folks that the best response to an active shooter situation is "Run, Hide, Fight." But the "run" part of that plan is more about getting to a safe place than actually outrunning the gunman. The safest place may be outside of the building, but it may also be in a locked room or office or behind some sort of barricade. That's where the "hide" part comes in. Meanwhile, "fight" is a last resort that should be implemented only when there's no other option [sources: FBI, Precision Risk Management, University of Delaware].