Is there a virtual online Hogwarts that you can take classes and that is popular enough that it has a lot of members

Yeah there's a sight called Hogwarts Extrme which is really good. I've never seen less than 70 people online at a time. You can "attend classes" and they also have several great rp forums. Anyone interested in Harry Potter rpin should check it out.

There is also Virtual-Hogwarts.org

Virtual-Hogwarts.org is an active online role-playing forum with over 9000 registered characters, that has been online since 2003, and stands as one of the most active canon-based RPG forums around. A short sorting form is all you need to register and begin your quest through seven years of magical learning and tumultuous adolescence. Get Sorted, take classes, Duel, play Quidditch, and even shop in Diagon Alley and Hogsmeade!