As far as I remember, there were дежурные (some children on duty) who stood at the doors of the cafeteria and sometimes were accompanied by a teacher,. If you were a slow eater (like I was when I didn't like the food) they wouldn't let you leave the cafeteria until you finished your meal. (Oh, those pancakes WITH sour cream and WITHOUT sugar were a nightmare for me!) And you couldn't just trash or leave your food either.. So they made you wait until the bell and then mercifully let you go (but now you were late for class). My mom still remembers many pieces of bread and hardboiled eggs that I brought home in my pockets, forgetting to toss them in a dumpster on the way home. :)