It has been a while since I have been involved in details (so this explanation may be lacking in the latest details) but generally those in the Prep School are enlisted personnel in active or reserve service (not coming out of high school or college) who potentially qualify for admission and have applied and been recommended by their unit commanders. However, they have been out of the academic scene for a while. These people usually require some care and brush up for the academics as well as further evaluation for final appointment. After a training period, the prep school then handles the appointments directly with Admissions. In the case of USAFA, the prep school is right on the Academy grounds and run by AF people, but completely separate from the main USAFA campus.
These prep school appointments are not done through the political system such as appointment by the President, Senator, or Congressman as the vast majority of other new cadet appointment are done. They are competitive based upon qualifications, final Academy needs before the year begins, and are done among the single prep academy group.
The demographics would vary with each prep school class according to who makes the cut to get accepted for that phase of training before appointment.
I hope that helps.
HS classmate of mine went to Army Prep at Ft. Monmouth. He was recruited for the West Point football team, but really didn’t have the grades or ACT scores to qualify straight out of high school.
Yes, getting enlisted people ready for the academies is one reason the prep schools exist. But the much bigger "real" reasons are (a) athletic recruits, and (b) diversity. Which of course overlap. For the football team, it is essentially a redshirt year. And a large percentage of black cadets are prep school graduates (whether or not they are athletes).