"The Prison" is what we nicknamed my All Girls HS (Catholic)
It was in Manhattan just a three blocks away from the docks. There were only two entrances: the front, and the back Fire Exit. Every single window, including the upper floors, had iron bars on them, which could not be removed.
The doors? The front door was a huge heavy metal double door with an iron deadbolt across the doors inside. There was no window in that, only a peephole. When these doors were locked, at all times while school was in session, there was no way to get inside, or out unless you were LET out by staff. There was an intercom outside the door to the office. Anyone, including students, would have to press the intercom (not buzzer), and speak to the office. Then someone would have to go to the door, look through the peep hole, and then unlock the bolt and open the doors from the inside.
The Fire Exit door could not be opened from the outside. That door also had a metal "bolt" inside, but could be pushed open from the inside in case of a fire.
A few years ago I took my daughters to see my old HS. It is still there, and looks exactly the same as it did back in the 60s.