From one of our first-year grad students -- in week thirteen of the semester, mind you:
"Yeah, Professor [X] hasn't gotten back any of our work to us, or assigned us any grades for any of it. So we've all just figured that either we're doing okay, or we're not, and we'll find out when he gives out the final grades... I guess he's working on his next book."
Ah. So that's the key to productivity.