They didn't redesign all planes after 9-11, they retrofitted pit doors so a code was required to enter. They ins uted rules of no standing or forming a line in the forward galley. When the pilot needs a piss break, the attendant stands guard in front when the pit door is open each time. In schools, the front doors are often unlocked, there's no load lock area where clearance happens before final entry to the school. There are no metal detectors, no one is standing guard, no canines and no reinforcements. There's nothing but glass and a hallway with a room full of children for a "my turn to go out in a blaze" dip to take advantage of. There's no egress routes that don't include passing through ingress routes. There's no backup plan.