Grading student art is the least enjoyable aspect of my being an art teacher. I don't write grades on the actual pieces of student art. In my mind I imagine a child proudly showing his/her art to family and friends, then some observant person will notice the grade written on the back. If its a low grade will everyone still value the creative piece? Will the child still be proud? Many families lovingly store their child's artwork for years, like keepsakes. An obsolete grade should not hang around too.
I tell my students what each project will be graded on. I don't say to them you're graded on "Workmanship" instead I say "Neatness." Younger students relate to "Neatness" better and I don't have to define it each time.
S+ = Proficient Level Effective Skill, strengths outweigh the weakness
S = Basic Level Developing Skill, strengths and weaknesses are about equal
S- = Below Basic Level Emerging Skills, weaknesses outweigh the strengths, some hint at understanding skills