There's a reason saferooms aren't really used outside of movies, especially with large groups of random, untrained people who aren't ready for emergencies like that. For the shooter, it actually makes a great point to hold and fall back into, as well as it conveniently gathering victims for them. Sure, no one can get in after it shuts (assuming it was made by someone intelligent, and built correctly), which tells me after the first handful of people get there they'll be terrified/panicking and such the door most likely anyway.