I am already admitted to the school. I don't want to look like I am brown nosing, but I do not want to come off disrespectful. I know I am over-thinking this, but humor me.
You're not overthinking this, as these will become increasingly important concerns.
Miguel is quite right that it's not a big deal here. However . . . I would tend to err on the side of a bit more formality on a first meeting--any first meeting. Nothing over the top in this case, but you might take it up a notch. Business casual is usually the safer bet.
(To any who think I'm wildly stodgy . . . if only you knew . . . think about this in a larger context. Chances are it will make absolutely no difference. It will certainly not make a difference as to admission, or grades, or whether the soda dispenser works properly. It might, however, make a difference when asking for a recommendation, or when your advisor . . . who happens to chair the committee for scholarships, or who is of counsel at the firm you REALLY want to get into, or . . . . See? It's a better bet to borrow a phrase from the military: "There's never a time you're not being observed.")