This is a drawing of a skull I did on top of the photo. This is the kind of correction I make. The students do all their drawings on tracing paper and/or vellum. On my assignment, I have placed  a number of registration marks. The following week, I will hand back to them the correction I drew. From there, they can simply place their drawing on tracing paper on top of my correction and correct their own. I used to go over each students drawing individually  but this was way too time consuming.