That is wonderful. Have you considered taking a few pictures of your chickens and putting them on the outside of the slop bin with the words "feed me"? (might want to cover them in clear contact paper or put the pics in a ziploc, so it doesn't get slop on it.
The problem with telling the kids that their unwanted food goes toward animals, is that they will then go out of their way NOT to eat the healthy food, and give it to the critters! Last year my mom, who works at the schools here, started collecting the apples and veggies that the students were throwing away. Once they figured out that she was collecting these unwanted, thrown away apples for my chickens, the kids that normally ate their apples an veggies started giving them up just FOR the chickens! We definitely didn't want to encourage the kids to get rid of their healthy food, so my mom started collecting trashed food more subtly!