It is hard to shake people’s perception that the top prep schools are feeders to Ivys. One, because some of them were founded for that express purpose. Two, because it used to be very much true. And three because it is still true that the very best prep schools probably send ten times more kids to the Ivys then the general population. However it is fair to ask “why” they get into Ivys, and it is worth noting that the odds are still not high.
This is an old article but it is very blunt and thorough and has an unusual wealth of facts about who gets into Ivys and why. The upshot … a kid who was 14th in his class at Groton was rejected from 5 Ivys, MIT and Stanford. A bit sobering for anyone clinging to the idea that a top BS is in and of itself is a an Ivy hook.
http://online.wsj.com/public/resources/documents/Polk_Groton_Grads.htm