Students at HYP are no different than students at UMich, UColorado or UCLA. Most buy their clothes from a retail store that can be found in any mall -- stores such as: Aeropostale, American Apparel, American Eagle, Banana Republic, Gap, Macy's, Old Navy, Abercrombie and Fitch, Hollister, J.Crew, Urban Outfitters, Century 21, Target, Walmart etc. Kids wear what they want -- not because they attend a certain school. You need to visit more than Ivy League colleges to see that all students dress pretty much the same, no matter what school they attend.
I just found this article that you might be interested in reading: The Fashion Choices Of Princeton Students - Business Insider