I was also a voice major in undergrad. How fun - but I TOTALLY get what you're saying about nerves!
Can you ask some other women in your studio what they're wearing? If the majority are wearing pants or dresses, that might be the way to go. A long dress with heels sounds a little fancy, to me, but might be appropriate depending on the culture of your music department.
I definitely agree that dressing well will give you more confidence. I'm sure you'll knock them dead, no matter what you decide to wear!