The lower levels look like a 12 yo drew it and the high levels look closer to Photo Realistic.
If i had to Define it on a 5 level scale (1-5) this is how I'd do it.
Level 1 - no detail, figure is distorted, no shading, avoids difficult tasks, no back grounds, only made up of sketchy lines.
Level 2 - Simple details, there's shading but lighting is off, some distorted pieces, finer lines, proportions are better.
Level 3 - a decent amount of detail, Has shading in the right spots, Use of foreshortening, simple backgrounds, still can see line but are well drawn.
Level 4 - Getting close to photo realistic but is still a little off, Shading and backgrounds are developing, starting to add texture, there is some life to the drawing.
Level 5 - HIGH Detail, Looks photo realistic, you can see texture in the drawing, the figures are well shaped, shading is perfect and you can tell where the light source is, full realistic looking backgrounds with use of blurring of foreground or background items.
For a non artist to really judge a picture it's really tough though... I hope my scale helps. If anything have visual examples of the levels and see where those drawings fit into.
3 example of drawings on 5 different levels (15 drawings in total) and play a matching game where the people's drawings fit in most. I say that would be the fairest way to judge them.