Back about 20 years, when I ran the school's computer lab, the emergency procedure was to shelter in place. Everybody away from the windows, lock the doors, and close the blinds. The walls were concrete and the doors were steel fire doors. I believe this is still the policy because schools are "locked down" if there's an incident in the area.

During classes, this is by far away the best policy since it secures the vast majority of students within seconds of the bell sounding. But between classes, the student body is vulnerable. A fire alarm may be beneficial then, since it puts the students in a situation where they can readily scatter. The downside is it takes longer then heading for the nearest classroom and they're still vulnerable to gunfire.