I am running Moodle 2.3 on my server. In Moodle version 1.9 you had added the "Revert to Draft" so that a teacher could send the assignment back to a student. I was working perfectly, however in Moodle 2.3 the process has changed to "Revert the submission to draft" and is accomplished in a different way than in 1.9. I don't know if it would be possible to add the new "Revert the submission to draft" on the grading screen.