Bah, we're screwed either way.
I was a freshman in high school when Columbine happened. This is when they started instituting the "Code Red" procedure, where if there was an active shooter/internal threat, everyone was to get in a classroom, the teachers were to lock the doors, and everyone stay down and away from the windows.
There were a few flaws in their plan, of course. All the classroom doors unlock from the outside, meaning the teacher has to step outside the door to expose themselves. Also, one part of the plan went "If you're in a hall, get in the nearest classroom." The next part (mostly directed toward the teachers) went "Lock the door and don't let anyone in." Anyone see a discrepancy here?
Of course, this procedure was only in effect for active shooters. We still did evacuation/fire drills for the off-chance of a bomb scare (and we had several bomb threats that year and the next.)
Of course, the administration wasn't too happy when I pointed out that if I were a shooter and wanted to maximize casualties, I would stand out of sight outside, call in a bomb threat, and wait for the people to line up for the slaughter. Like most people, they did not like having the holes in their plans exposed. I do feel bad, though, because you're screwed either way. There's no easy answer to any of these questions.