This seems to assume that all students are “discarding” the same quantity of items each year. It also assumes that the only student donations that occur are ones that are tracked by their university. It’s hard to believe that it is true.
A place like UChicago is not known for being a party school; I doubt Balenziaga or Valentino items are in high demand. I would assume that people aren’t all that into fashion that goes out of style quickly, thus they probably aren’t throwing as much away. But maybe that’s just an “unfair” stereotype I have about UChicago students ;)
One thing I do know, however, is that up in the area of Northwestern, there is a strong tradition of donating things to churches and synagogues, who then hold rummage sales. There is even a “rummage sale season” and a circuit - every weekend there is a different set holding sales. It seems that any such donations here would not show up in any data that this author has collected.