Where does uneaten school lunch go?
The answer changes from place to place. Sometimes it is burned as trash. Some schools recycle the food instead. It can become food for pigs. It can also become compost. Compost is food for the soil. Some schools use it in gardens. (mext.go.jp)
Think of this example. A child leaves some carrots at lunch. The carrots go into a special machine. Later, they become compost. The school garden uses the compost. New tomatoes grow in the rich soil. They may become part of another lunch. Old food can help make new food.