In my experience, prep schools have a higher proportion of Asians than would be expected based on the proportion of Asians in the US population. Of course I'm not adjusting for things like income.
It's also important to look at the ratio of applicants not the general population. Further, schools which give preference to 'legacy' students whose parents went there are not going to reflect more recent trends.
http://www.usaschoolinfo.com/school/phillips-exeter-academy-...