Generally it's khakis and a golf shirt, or scrubs (depends on the clinic and personal preference). Khakis or shorts in classes (1st year). Doesn't matter so much as long as you look reasonably bit professional.
Fortunately they are (or were for my class) pretty firm about the ladies not wearing short skirts or necklines to their waists. Same for guys wearing ragged t-shirts, etc. Again, about looking professional.