Alfie Kohn:

But if you read the results rather than just the authors’ spin on them — which you really need to do with the work of others working in this field as well[7] — you’ll find that there’s not much to prop up the belief that students must be made to work a second shift after they get home from school. The assumption that teachers are just assigning homework badly, that we’d start to see meaningful results if only it were improved, is harder and harder to justify with each study that’s published.