gents:
maybe 1-2 weeks worth of slacks/ khakis, ties, and dress shirts. you'll most likely have to wear slacks, shirt, and tie (+ white coat) for the days you're in the clinic/ hospital. for the days that are only lectures, i presume that casual is fine, unless your school is really picky.
ladies:
the equivalent of shirt/ tie (usually "relaxed business" / "business casual") over which you can wear the white coat.
and, of course, scrubs for the cadaver labs, OR, ER, and other more "invasive" clinical experiences