Sorry, but you're full of shit on this. For one thing, Dartmouth is an excellent college and Duke Law is an even better law school. Anyone is doing pretty well if that's their course in life, regardless of the prep school they went to. So just from the beginning, you're starting with a really shady premise.
Looking at numbers, Dartmouth is one of the more common colleges that Exeter grads attend. Yes, a lot of them go to higher-ranked colleges like Yale and Princeton. A lot of them also go to lower-ranked colleges like Tufts, Carnegie Mellon, Cornell, Michigan, etc. A bunch more go to small liberal arts colleges like Williams and a lot I've never heard of. So unless you're suggesting that like half of Exeter is a bunch of fuckups that are wasting a guaranteed seat at the Ivy League, MIT, or Stanford, this doesn't really hold water.
This isn't like back in the 50s, when going to the right Boston private schools meant you were literally trained to pass the Harvard entrance exam. Exeter is a very good prep school. It gives its students a lot of opportunities. But it's ridiculous to say that because of that, if you didn't get into one of the top six universities, then one of the top six law schools, you've somehow fucked things up.
You could look at rankings alone and say that since Exeter is such a highly-ranked private school, it represents some kind of failure to not go to equally highly-ranked universities/law schools. But first, that would be incredibly douchey. People in the real world don't usually give a shit about numerical rankings, and they will absolutely look down on you for actually basing your personal judgments off of them, especially when we're talking about institutions that are all top-notch. Someone at Princeton is likely to be more intelligent than someone at Arizona State, probabilistically speaking, but it's a lot less justifiable to say that someone at Duke Law is dumber than someone at Chicago Law.
It would also be unrealistic. US News and World Report (whose rankings I used for reference to colleges and law schools) doesn't actually put Exeter anywhere near the top of its list of high schools. Exeter is not the be-all and end-all of high schools.
tl;dr -I suppose my answer to the question in this thread would be: the kind of uninformed elitism you're displaying.