A lot depends on the candidate, why you were rejected and if prep school will correct those "deficiencies".
I can add this link to the Service Academy Prep program at MMI:
http://marionmilitary.edu/programs/programs-sap.da
You need to apply and meet certain academic criteria to get into the SAP program. They do have a very good track record and kids who were not sponsored do win appointments.
You also can choose to stay for two years and then transfer to a 4 year. Many colleges will accept the credits.
If you are AF they also have an AF ROTC program in which as a SAP cadet, you will participate. MMI has an excellent relationship with the AFA and will provide you with support and counseling.
I would choose programs wisely. If you have an AF ROTC scholarship - think twice before giving that up to go to a "prep" program. If you want college credits that transfer then consider the Junior college route.
You also might want to talk it over with your Admissions counselor at the AFA.