I would hazard a guess that it has to do with a lot of American schools being made during the Cold War, and designed to double as emergency shelters.
Some/all of that have less to do with war and more to do with general disaster preparedness.
* Bomb shelters provide shelter from...bombs, yeah
* Windowless hallways provide shelter from tornadoes (no glass to break/become shrapnel)
* The heavy metal doors are firewalls that automatically close when the alarm goes off (most are hollow, won't do shit against assault weapons and predate even Columbine)