For Orientation, I might call and check to see if you haven't missed something. At Emory, the first day is casual, which is basically wear whatever you want. Second day they specifically said "business formal (aka a suit)." We have photos taken for the facebook and are sworn into the law school. Each school is different, I'd imagine, and many don't require a suit at all.
As for once classes start, it seems pretty universal that you wear whatever you're most comfortable in. If you like dressing nice, then do it. If you want to wear jeans/short and flip flops, then go for it. I, for one, plan on pulling out my hooded sweatshirts once it gets cooler here.