I'd be interested to see if you could provide a reference for that statement. My experience with undergraduates I've met at MIT and Harvard, for example has been that they are predominantly middle class - however with still predominantly more upper class students than working class. Of course, that's just my subjective impression.
The bulk of Ivy League undergraduates come from white-collar professional families. You can quibble about whether that's "upper middle-class" or "middle-class", I suppose, but you don't find most of them at the yacht club.
Interesting. I studied low at Harvard for two years in the middle 80s. Around 75% of my colleagues were from "yacht club" families. Maybe the things have changed since.