Sounds like an opportunity to enlist some form of alternative assessment.
I have the same problem but have somewhat struggled to come up with a
solution that works for now. (If truth be told, I'd get rid of all grades
in elemetnary art). However, as part of our system assessment, the art
teachers created a (rating scale/rubric) for a given project. There is a
sentence that specifically focuses on following directions etc. It has
nothing to do with the assessment of the project. So a student could in
fact reach all of the levels of acheivement but still not follow
directions. These are attached/stored in their portfolios and go home at
the end of the year. Let me know if I can clear anything up.