Good inexpensive art classes in San Francisco?

I’m looking for a place to take a good watercolor class, without having to commit too much time (i.e. I can’t take a professional class that meets three days a week). The class would have to be outside of 9–5 hours, preferably on a weeknight. And I’m looking to find something that starts relatively soon. I don’t care if I get any type of academic credit; just looking for something that will help me be creative. Watercolor would be fun…other kinds of art classes would also be interesting to me.