Here are several plausible explanations:
- Disadvantaged children maybe required to do other responsibilities outside schools by their parents, so they tend to constantly seek unstructured time
- Middle-class children will have more time with their parents, while disadvantaged children's parents may took two jobs-
- Middle-class children tend to live in a more structured neighborhood that lead them to spend more time in structured activities while it is very likely that disadvantaged children had to grew up in a chaotic society.