Here is an excellent article about what teachers should do when homework is turned in and it is incomplete. The tempatation is to give the student a low grade or you can have them complete it. I have found that if you don't accept incomplete work, they will most often get it done. The same can be said for poorly done work. Would you as a parent just let your kids off the hook if they didn't complete all of their dishes? This post does a good job explaining why students should complete all of their work.