For a lot of kids those school lunches are their only access to balanced nutrition. Parents could be very poor, ignorant or malicious.
Especially in a house where they might eat mostly grains, and the occasional peanut butter or hotdog. Or sometimes nothing but water, and bread.
School cafeteria often let kids rack up some debt and try to bill the parents later, or for free you get a bagged lunch with like a turkey sandwich, fruit and milk, maybe a snack, maybe it's peanut butter sandwich instead of turkey, especially for vegetarians.
School lunch was usually lit. The kids with money would leave stuff they didn't eat near the tray washing kitchen.
Edit: I forgot to mention, the left stuff was usually closed milk containers and unopened packages or fruit that hasn't been bitten into, it wasn't gross.