The waste is horrific. I have extended family working in the schools, and they all say that what these kids toss away every day would feed a small Third World country.
You brought your empty tray to the cafeteria window to trade for a desert. Uneaten food on your tray meant no desert.
Of course, we learned how to stuff a lot of icky food into an empty milk carton to give the appearance that we'd eaten all of our food. But there was a limit on what you could stuff into the empty milk carton. And you learned to drop it into the trash bin ever so gently because if there was a tell-tale thud, the lunch ladies would fish it out of the trash, open it and dump it on your tray and tell you to either skip desert or finish your food.
They weren't squeamish about doing it either.