I can't speak directly about New York or the late eighties BUT, I went to a 'big city' high school in the early seventies and my son attended high school not that long ago.
Basically a closed campus isn't actually locked although it might be fenced. Students with cars could (and did) leave campus by simply avoiding the 'monitors' (aka wardens). Seniors were frequently given more leeway - partly because many of them were close enough to 18 for them to claim they were legally adults.
On my son's closed campus high school, the juniors and seniors with cars went across state lines to go to lunch. The ring leaders at the time were the sheriff's daughter and my restaurant critic son.
(And yes, the kids I went to school with and my son thirty years later all used prison terms to describe the staff.)
