One class at Virginia Tech that had no losses was due to a girl getting up as soon as she realized something was wrong, carefully peeking through a crack in the door, then closing it and getting help to barricade it.
Like burglars, terrorists like school shooters will probably follow the path of least resistance -- if one door won't open, they'll try another. They are also probably quite confident that everyone will panic or run, not attack.
IOW, have some kind of a plan for as many possible situations that you can think of: attack, run, hide. Then you might possibly have something to fall back on if it happens, rather than just having your brain spinning in neutral.
But I guess in a geology class you could always throw rocks!
Do you know if your school has any kind of plan? What about some of your classmates?
Sue