In the US, in the 1980s onwards, they redesigned universities to be able to crush student protests outside the building. many wide entrances make them hard to hold, a lot of other design choices.
After 2005, schools were built, designed, and retrofitted for a physically-weaker, small force to be able to hold all the exterior doors, and failing that, every hallway door, against a motivated opponent with more tools than them.
and public schools have been teaching Gen Z the skills of how to defend those buildings. Drilled into them with trauma, year, after year, after year, after year, after year. Because of the threat of shooters, and no one actually dealing with it.
They made the outside police friendly. they thought they made the inside police friendly. Because they never thought the students might not side with the police. Because it never was a serious solution to the shooter issue.
edit: I missed @alyaza's writing on this yesterday, check it out: https://cohost.org/alyaza/post/5721657-the-student-protest which is a debrief/how to, and looks like they were informed by Occupy. but the OP's insight that they've built and trained for this accidentally is still huge and hopeful to me.
like. fuck.