Lets clear a couple things. I think when the OP said "Transfer Rate" they meant as in: "What are the percentage of prep school students that get to go to the academy". Well basically; as long as you pass all your classes; don't get into trouble; then your odds are extremely good to get an appointment to the academy.
Now; one thing the OP said that needs to be clarified. YOU, don't get to CHOOSE whether you want to go to the air force prep school or not. You must apply to the air force academy; and if you don't receive an appointment, the academy MAY OFFER YOU the opportunity to attend the prep school. Normally, the prep school is for those that except for a minor academic issue, are otherwise considered extremely competitive candidates. And it is a little tricky. If for some reason you had a 3.9gpa; excellent test scores; etc... but you didn't get offered an appointment; "Possibly because of a lack of leadership, athletics, etc..."; then chances are extremely slim that they would offer you the prep school. The prep school is offered to those who have the leadership, extra curricular, sports, athletics, and generally speaking very good academics. They just missed out on the appointment because of a particular area. E.g. 3.85 gpa but a 25 ACT. Or; a 36 ACT, but a 3.4gpa. Either way, the point is; you don't get to choose to go to the prep school. You apply to the academy; they decide whether to offer you an appointment; the prep school; or nothing at all.