Well, @sunnyschool, I’m not an expert on all BS of course, but I think it is a combination of factors that leads to relatively low percentage of NMSF at almost all BS. The course load is generally tough, the kids are busy with a lot more than academics, and I think there is a conscious culture of “not teaching to the test” in general (you can see this in how some schools - such as SPS or St. Andrew’s in Delaware and there are others - take pride in not offering “AP” classes).
However, I think the primary reason is that boarding schools are looking to admit students on a wide range of criteria, only some of which have to do with intelligence or academics. So, I think on average the student bodies at even the “best” boarding schools are not as smart - again, on average - as the students at the “best” (and perhaps even the merely “very good”) public schools. It’s just impossible to square the data otherwise.
For instance, top magnet high schools regularly have NMSF exceeding 10% of the class - even some open enrollment good high schools in places like Silicon Valley or suburban Boston have numbers that high. The most selective magnet schools, and private schools that screen for very high intelligence (e.g., the NYC elite high schools, parochial Regis High School in NYC, Thomas Jefferson High School in Virginia, etc., the private Harker School in CA) can have numbers that regularly exceed 20% of the class. And I can assure you there is no hand holding or PSAT test prep at those places.
It’s basically who you accept that determines the numbers. At the extreme, Davidson Academy in Nevada, where they can only take kids who are at least 3 standard deviations above the mean in intelligence (top 0.1 - 0.3%), the NMSF percentage is approaching 70-75% most years (although they are helped a little of course by the easier selection in index in Nevada, as opposed to California, Massachusetts or NY).
It is hard to find numbers for most boarding schools, but from what I have been able to gather, PEA regularly has 10-15% of the class NMSF, PAA about 8-10%, Choate maybe 4-6%, and the rest lower, with many even “elite” places like Hotchkiss and sometimes Cate and Thacher generally having no more NMSF than one would expect from a random draw of PSAT takers across the country. I for one would appreciate if people could post actual data, because as I said the data are not easy to come by. Hope that helps!