I don't think it was established that he was attending clothed or nude...or that she either did or didn't teach classes to clothed mammals.

If she teaches clothed classes, I'd have to question why Emmitt didn't attend those instead.

That would be my guess as well.

Of course this one deserves an answer from Byron Howard (as each question so far does), but I would venture a guess that the Mystic Springs Oasis is more than just a nudist colony...that Nangi, the elephant who seems to be in charge of it, also teaches yoga, meditation and other classes...perhaps also to fully-clothed citizens of Zootopia.

Of course, that's just a wild shot in the dark. It will be good to see Byron's response on that one.