I went to high school in the Bronx. No one ever left for lunch. People either brought theirs with them or ate cafeteria food. Almost no one had a car and you needed a car to make it to the local food places and get back in time to get to the next class.
Also, the front doors to the school would lock and the receptionist would have to buzz you in if you left. I don't think it was really against the rules to leave for lunch, but no one ever did. The most we did was sometimes eat on the lawn in the backyard. (My school overlooked the water - you could see Manhattan across the way.)
