the essays posted by students who were admitted by top prep schools

Prep School Admissions Prep School Parents
2

http://talk.collegeconfidential.com/prep-school-admissions/869050-post-your-essays-for-future-applicants-p1.html

This might be what you’re looking for. It takes a bit of sifting to find the essays themselves, but there are some truly phenonmenal pieces buried in there!