If you look at the coven profile pages, most do are our currently offering classes. My suggestion would be to find a coven you like and apply to it to view the offered classes. A lot of classes are offered in the coven's forum and you can easily read over any old posts for that class. Other classes are taught in coven chat, so you would have to speak to the class instructor for a refresher on what's already been taught.
Some covens advertise that they have classes, and some actually do have classes. It's just a matter of finding the one suited for you. Most of them are pretty helpful. The coven forums in general usually have loads of information floating about. And if the coven has been there for awhile, you can browse to the very back, see years of discussions.