J.H.
If my 10-year old son came home with this problem, honestly I would email the teacher, but just to say that he says he has this problem and could she please keep an eye on the situation. Then I'd probably tell him to stand up to the girls. If they call his artwork trash I'd tell him to respond that he doesn't care what they think, adding a "that's a really mean thing to say" in a condescending tone (teaching him how to say it that way). And I would probably have him continue to mention to the teacher any time he is stuck doing all the clean up. But hopefully bringing the situation to the attention of the art teacher from a parent would help the situation
I'd probably also copy his main teacher on the email too.
I'm all for letting kids navigate their own way socially, but I know if it was my son this would be more than he is equipped for right now and he definitely needs the assistance of adults. Sounds like the girls should be split up to different tables.