Very well written explanation and description.
It is interesting to compare the situations in the East Coast, where private prep schools are at the top of the pile to the Chicago area, where the high schools with the best reputations are the public magnet schools. These schools do not have a legacy and athletic admissions, so they do not have the large number of legacies for “elite” colleges, nor do they have the large number of athletic admissions to “elite” colleges. So most of their admissions to “elite” colleges are based on the accomplishments of the students, the reputation of the school, and these connections between the school and the colleges.
For example, Walter Payton in Chicago has around 30% of the students at Payton are considered low Income by the state of Illinois, meaning that they get discounted or free lunch, or their parents are on federal or state aid, so bottom 20%-40% by income. For comparison, “elite” private prep schools have 30% from the bottom 80% by income. While the income distribution is not comparable to that of the city, it is still far from the mostly wealthy students at “elite” private prep schools. That means that there are far fewer legacies and potential development admissions. Since Payton doesn’t consider legacy of athletic ability for admissions, this reduces almost all athletics admissions to colleges, as well as another large percent of potential legacies for colleges (a large number of “elite” prep schools legacies are also legacies to elite colleges).
Since admissions to Payton are very tough, and no students get a nudge, students are all pretty good.
So, without athletic admissions, a small number of legacies, and very few super wealthy families with connections at “elite” colleges, Walter Payton has about 15%-20% of its graduating classes ending up at “T10” colleges, and another 10%-15% at “T20” colleges.
That is probably what one should expect from all of the “elite” private prep schools, once you remove legacies, athletes, super wealthy and connected families, and such.
However, the connections between GCs and AOs still exists, and is clear when looking at the patterns of attendance at low admission colleges.
For example, and very large number attend NWU, but not equivalent colleges in other parts of the country. More attended UChicago than any other colleges with similar acceptance rates. The number who attend MIT, Harvard, and Yale is strong, but very few attend Princeton. Similar patterns can be seen in matriculations to LACs - Pomona has many more Payton graduates than it does Williams, Amherst, and Swarthmore together. Very few attended Notre Dame, since Payton is not a Catholic high school.
So schools like Payton will tell us what the real boost is for unhooked kids who attend an “elite” high school.
I would expect though, that the “elite” east coast boarding schools have an additional advantage. Thy have decades of connection, and generations of there being a pipeline from these high schools to the Ivies and other “elite” colleges. This benefits unhooked students as well these days.