So yes, there’s enough truth to go around on this one.
As @OHMomof2 notes, at most of the top prep schools 40-50% of the class may be getting financial aid. And at least based on the families that I know, if you needed FA for prep school, you need it for college. Just like everyone else, they are choosing based on affordability. (And this is one of the reasons, when you look at prep schools to see where kids matriculated, that you’ll see a bunch of good, but not totally tippy top, schools in that list. Trust me, this is generally not a reflection on the quality of kids in those schools or the quality of the education but an indication of how committed that school was to taking kids who couldn’t write a check for full tuition. In other words, it’s just like the rest of the world!)
On the other hand, the other half of the class is full pay. And many of those kids, as @Lindagaf notes, are from VERY wealthy families. Many will have had all the advantages imaginable, from tutoring to travel to whatever. I think that many prep schools DO tend to give some thought to college admissions when they are admitting their freshman class. The kid who is already an athletic stand-out, whose parents are big donors to HYPS schools, who has won international math contests – in other words, the kids who will have hooks for college admissions – is attractive mostly because he/she will be interesting to have in the community but also because he/she will be easy to place in college…
But definitely, it is not all wealthy kids…