This means that the students still have to take on board the comments you make.
So simple! Why hadn't I though of this before?
Another suggestion was that you prepare example pieces of work, a high, middle and low. Then get the students to compare their work against them. Ultmately placing their work on the continuum and being able to explain why it would fit there.
This last suggestion is certainly something that I will try after the half term break in order to try and get my students to improve their writing.