"What changed from as little as 20 years ago when it was perfectly legal to have guns in your car at school?
"
The laws, obviously. I dunno, but I seem to remember when those school gun ban laws were being written.
I'm not sure that minors are covered under the 2nd amendment. Most states have some laws about firearms possession by minors. It varies from state to state.
The laws don't apply to only minors.