Why are all the schools like prisons?

No. Back in the day, school was a release for children from working in sweatshops.
There was little to do at home, so school was felt to be a great freedom for many children.

Imagine you live in a tenement or apartment. No yard. No swings. No television or computer of course. Few books. Maybe an old radio.

so kids loved school. It was better than home.

Fast forward: home is infinitely more alluring than school. When you have all you want at home, school is for socializing only.

some kids socialize nicely. Others brawl, fight, cuss, scratch, and worse.

Hence the need for police in schools. Hence the prison feel.