This is a really simple lesson plan, but it introduces ciritcal thinking skills like analysis, inference, and interpretation, which are valuable in any subject. It doesn't hurt that a lot of the answers you get out of this exercise are also pretty funny too.
Cindy Foley, Executive Assistant Director and Director of Learning and Experience at the Columbus Museum of Art, discusses how art isn't just about art, it's about encouraging and developing creatiivity, which is a highly sought after qaulity in the adult workplace.
I like to think of this one as a jumping-off point for more research. Here, ten important studies on art education are briefly summarized, which will give you a good foundation, but the links to the actual studies are also provided for a deeper look. Follow the references in those studies to fall even further down that rabbit hole of knowledge about the arts in education.
The National Art Education Association answers the question of "why teach art?" in a succinct and informative way. This one will be handy if you ever have to address administation that wants to cut arts funding.