This is debated heavily from time to time on the prep school threads, and it may be a matter of semantics, but one of the main reasons prep school kids reliably get into top schools at a rate that is disproportionate stems from the fact that the prep schools themselves are highly selective, many taking under 20% of their applicants. It is true that if a kid is an academic standout at Lawrenceville, for example, no tippy top school will doubt that student’s ability to do the work. At the same time, that kid may also have 30-40 really amazing, as in equally viable, classmates (including athletc recruits who did a PG year) who have applied to the same school. So the prep school may be able to report 10 acceptances to Princeton, but 20-30 very, very good applicants still were rejected. But from the outside, 10 kids to Princeton alone looks sort of amazing. Know though, that at all of these schools there is a small cohort of parents every year who are unhappy with the college outcomes for their kids, and it’s one of the reasons that all of the parents on the prep school thread reiterate that you should choose prep school for your child if you think it’s the best place for them for high school, not for the four years after that!
Certainly, adcoms know about the quality of many excellent public schools and their students, and the success at @Lindagaf’s school and her daughter shows that. With that said, the depth will not be the same at a public because the public doesn’t have the luxury of choosing only the “top” 20% of the 8th graders (and not just from its feeder middle school ) and “not inviting back” or “counseling out” students who are struggling with the breakneck demands of those schools over the 4 years.
I’m not disagreeing that prep school kids are “over represented” at many top schools, just pointing out that admissions comparisons are tricky and the perceived “preference” may not be quite as clear cut because the pools can be quite different.
It is true that prep schools almost always offer better college counseling and support, and there is a benefit in that for sure, especially for the kid who is “only in the middle” of his class at Exeter (and whose parents are wondering if he’d have been better off in the college admissions game staying at his public school, where he’d have been at the top of his class… I’m guessing probably not, but who knows?)
And as @proudpatriot points out, no matter which classes one chooses at prep school, it’d be hard to select a path that would “look bad” on a college application. Some may look better to some colleges, but overall, all roads lead to college. But then again, they’ve selected kids who were probably happy to be on that road.
As in so much, there’s a huge "it depends " in all of this. The process is really competitive and somewhat random with both students and colleges trying to evaluate each other based on data each has curated to appear as attractive as possible to the other. Speed dating with consequences! The reality is that there are a lot of great schools out there, and a good student will get tons of value out of any of them regardless of where they went to high school.