Unfortunately, I think many younger studio staff instructors have decided that the way to break the stereotype that dancing is for over the hill sqaures is to dress like dropouts and slobs, thinking this makes them look artistic and cool.
As a general rule, you should always be as well dressed (or slightly more) as your audience - your interviewer now, your students if you get the job. In the case of dancing, you have some latitiude to decide where you want to be on the spectrum from formal to athletic, but realize that jeans are neither.
I like Laura's suggestion of flowy pants. One nice thing in women's atire right now is that a suit-like jacket can be worn over a whole variety of tops which would give you a lot of flexibility to adjust your image after you arrive - if you feel overdressed you can take the jacket off immediately as if it were only for warmth outside, but if things are more formal than expected you keep it.
Also, in term of your footware, if you are going to wear something unsuited to proper dance performance for the interview, then you should probably bring a pair of dance shoes along - not that you will likely wear them, but you should be seen carrying their bag (which is hopefully in good condition, not that ratty one your practice shoes live in). It's really hard to take a dancer in say clog sandals seriously.
