I'm not questioning your own validity, I'd just love to know how pervasive this was and where it was happening.
I was able to find references about Justice Scalia in the Heller arguments.
I have no idea of how pervasive it was outside of Joplin (MO) High School in that period, but given that Joplin is somewhat urban, with a population of 39,000 then, I'd expect it to be very widespread when you add more rural districts.
Anecdotally, growing up in Northern Wisconsin in the 1980's, storing guns in lockers would not have acceptable, but this was a non-issue because many students drove themselves to school and would keep their hunting guns in their car rather than bringing them inside. It's possible this it was officially forbidden to have a gun in the car on school property, but in the absence of other circumstances wouldn't have been any cause for concern.