What does "no value" mean in the classroom?

I am constantly lecturing my drawing students to perfect their value scales and to understand the graphite tools from 6B to 2H at their disposal. We have been in school four weeks and, after finishing still life drawings ,they were allowed to pick their own-source photographs to work from. These are some of the free drawings they have tackled.

When I say "the work has no value", I do not mean that only their grandmother will like it. I mean that there is not enough variation in lights and darks. These kids are getting their lights and darks into a good place and making a very happy teacher!