The federal government provides money to schools so that schools can provide low-cost food in their the cafeterias. The food provided has to meet nutritional standards ( https://www.fns.usda.gov/nslp ). The federal government also sends some actual food to the schools ( https://www.fns.usda.gov/usda-fis ). Honestly, even with the subsidy and supplies, it’s hard to imagine how the schools provide much food.

Students from low income families can qualify for completely free lunch, and others can qualify for drastically reduced prices for lunch.

I don’t know if there are any federal requirements about how to handle students who forget or lose their lunch money, or students who normally bring their lunches to school but somehow forget or lose them. Most schools will have a specific policy, such as providing the food to the student and requesting the money later. The “debt” in question is between the school and the student, not between the school and its suppliers.

There is a relatively new policy (maybe ten years old) that schools with a large number of low income students can provide free breakfast and lunch to all students ( https://www.fns.usda.gov/cn/cep ), so school lunch debt would be an issue in areas doing just a little better than that.