In their view art education should be championed for its own sake, not because of a wishful sentiment that classes in painting, dance and music improve pupils’ math and reading skills and standardized test scores. (You can read the article here.)
Ellen Winner and Lois Hetland of Project Zero have a very good point. If it is desirable to increase proficiency in math, why not have more math studies? They seem to be championing art for its own sake instead of as academia's lame sister.
I have worked in arts education projects and community art projects for a number of years in several capacities. And you know? I can't really say what the long-term impact of these projects are. Typically, my contact with students is pretty much one-time only. I don't receive reports on changes in the students' achievement.
I think that where artists can contribute in educational settings is in projects that demonstrate synthesis. Arts studies can demonstrate the application of abstract knowledge. This type of project leadership is rarely undertaken, however. It can be complex to administrate. It is difficult to demonstrate the outcomes.