You can really wear whatever you feel comfortable in, whether it’s a skirt/dress dealie (regardless of your gender), jeans and a nice (or not nice) top, etc.
I’m pretty sure I wore something ill-fitting that made me look like a frumpy junior office manager. I also arrived to my interview more than an hour early and sat in the parking lot freaking out because I was (am) nuts about being punctual.
When the interview finally got started it took me about 5 or 10 minutes to relax and realize that no one was going to ask me a question like “What is your greatest weakness?” or “Why should you be accepted to Bennington?” Instead the person who interviewed me was incredibly nice and just wanted to know about what I was interested in and liked to do.
If I could go back in time I would have worn something that made me feel good (in my case, garish red lipstick) and tried to relax a bit and enjoy the opportunity to have a conversation with an admissions counselor (all of whom, I can now vouch for, are quite awesome).