I once saw an elementary report card for art that was pretty neat. It was
actually more of a checklist for developmental levels. For each skill there
was a rating that gave a truer picture of each child. I wouldn't recommend
it for 500 students a year, but perhaps something could be included in their
regular report card.
Skill not yet getting there doing well excellent
Understands primary colors
Shows interest in art works
Put in whatever you need to. No grading, but a picture of what the child
can actually do. As a parent, I appreciated the report cards in this format
and knew what we needed to work on or explore at home.