My school had these too. The code they'd say through the PA system was "Mr. Rush is in the building" (GSP?). It meant there was a possible violent person in the facility, and just like Roflcopter and the story said, we all locked the classrooms and kept away from the windows.
They'd do the drill 2-3 times a year and 1-2 of them would be to search for drugs. You'd have to be pretty dumb not to know the real reason as the teachers/staff openly admitted it and all the students expected it. You'd hear "Mr. Rush is in the building" and all the more "devoted" stoners would get pretty nervous.
Story is making something out of nothing.
sig credit to Limba