The high school I attended in CT (Amity) had locked doors except the
main ones visible from the main office area. All the other doors and
ground floor windows were locked but it was to keep students *in* not to
keep outsiders from entering. I used to skip the last period study hall
all the time to go to work at the garage I worked at after school. I
didn't try to escape from windows or locked doors. I just walked past
the main office with my books under my arm, waved at the office people
and walked out the door. It's amazing what you can do if you act like
you are supposed to be doing what you are doing. No one ever challenged me.
The main building of the high school I attended in SC back in the 70's was shaped like an "H". There were at least 4 doors that were unlocked all day and you could come and go through any of them. The ones at the ends of the H had no windows directly facing them and couldn't easily be seen by any teacher or the office. For a little while early in my freshman year, I was dropped off at school early in the morning before very many students or teachers were there. I walked up from the parking lot and entered through one of those doors. I freely roamed the halls, put my books in my locker, and wandered back outside to wait on my friends.
Little doubt that things are very different today.