Hmm, I totally agree that the art teacher is perhaps holding to a line in the sand that may not apply to your son. But, from the other side, if this is an art magnet school, can it be assumed that the level of art produced by the students may be above what would be expected in a normal 3rd grade art class? If so, in her mind the skip is probably not in the best interests of her class or your son. His art skills are not above average for his age based on the cousework she is teaching. Added, to that is that her student (your son) is not completing the assignments when he is in class and is missing even more classes. Many teachers would fail the student out of hand, rather than give a not graded score.
I think Kriston hit it on the head to take the skip out of the discussion and talk about what can be done to help a struggling student.
However, your son like mine is not helping his case by not completing his assignments when he is in class. My son is subject advanced in Math. This means that he is expected to meet (or exceed) the bar of the fifth graders even though his fine motor skills and organizational skills are below his chronological age. His teacher does not make any exceptions for him just because he is younger.
I certainy wish you well, and hope that you, your son and the school can find a meeting ground to help with this problem.
