by Davo Smith.
Once you've upgraded to Moodle 2.5 (or above), the 'resubmission' feature becomes available. This allows you to allow resubmissions on assignments either manually, at the teacher's choice, or automatically if they haven't yet reached the required grade.
The full history of all attempts (including the feedback given at each stage) is retained for each student.
There isn't, as far as I know, a way of automatically 'reverting to draft' with earlier versions of Moodle.