Well, you should be able to find references to kids that continued this sort of thing, storing them in their vehicles, who got sweep up in the post-Columbine zero-tolerance bullshit. I know I read more than a few of those, and to this day kids still accidentally forget and leave their hunting gun in their vehicle and get punished.

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.

I'd guess was fairly common in the rural US anywhere that an after-school hunting trip was common. This has probably always been a relatively small but non-zero percentage. I wouldn't be surprised if it still occurs in some communities.

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.