I'm confused -- you're getting interviewed
after you've already been accepted? What is the purpose of the interview?
Since you're already accepted, I would not "dress up" as much as I would for an interview that would determine whether or not they were going to take me, but I still would not go any more casual than "business casual"-type attire -- dress slacks with a nice, tailored top (and I, personally, would wear a jacket, but that's just me), "sensible" shoes. No jeans, t-shirts, sneakers, cleavage or midriff showing, etc.
You can always dress more casually once you have a better feel for the general atmosphere of your school, but it would be a shame to
start off on the wrong foot.