Edit: Fixed some misunderstanding in my comment.
You have a balance it charges against. In theory, when it hits zero, no more food for you.
But that's impractical because kids are bad about just about everything, and so denying food because they forgot to remind their parents to recharge the thing is harsh. So the system lets them go into "debt" and still get a lunch.
And it doesn't really stop, but the school will often put up a bit of a fight over final grades or diploma to try to get the debt paid off, but often it just gets written off.
But yeah, the problem you point out — families can go into debt again — is real, I think.
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.