I was wondering what would get you an appointment to a prep school, such as NMMI, as an alternate to an academy.
Are you talking about the "Civil Prep" or foundation scholarship program?
The Alumni Assn at USNA and USAFA and the Assn of Gradutate at USMA sponsor a program and award scholarhip dollars to candidates recommended by the Admissions office.
These are candidates "who missed the cut", usually for an academic reason, who have excellent athletic and leadership portions of their packet. If they "missed the cut" it may be because they come from a highly competitive district, or a little bit weak in English or Math. Most kids who attend give up excellent admissons offers at other civilian colleges and universities. Attending a SA is by far the first choice for these kids and they will spend an extra year in school if that is what it takes.
The academies are willing to take a chance on these candidates because they are excellent candidates. You are given a choice of different "prep" schools - dependent upon the academy.
West Point requires it's AOG scholars to attend a Military Junior College (except in some recruited athlete situations) and MMI and NMMI are the most popular.
Both NMMI and MMI offer a Service Academy Preparation program for students applying to all Service Academies. Most of the kids in the programs are Sponsored cadets but a few are there on their own.
The benefit of being sponsored is that Admissions is basically holding a seat for you if you satisfactorily complete the program. Other candidates compete against the rest of the applicant pool.
If you are applying to an Academy - and you have already decided to reapply if you don't make it the first go-round then let admissions know you are willing to consider a prep program.