Not that it couldn't have changed since I left, but when I was there, business casual was only required during clinics on specific rotations. Otherwise during the lecture years you can 100% wear what most people would consider casual- I normally wore hoodies and comfy jeans with sneakers to class. Others wore jean shorts with a t-shirt or a blouse. If you looked at our class as a whole ~95% of people wore jeans and a t-shirt most days. The only thing they were sticky about were PJ bottoms, yoga pants, scrubs, or athletic short-shorts. When you get into clinics there are some rotations that require business casual, though others have you wear scrubs or coveralls. Thankfully each rotation has those expectations lined out for you before you get there so it's pretty easy to know what you have to wear.