You cannot apply for the prep school; it doesn't work like that. If you are deemed someone WP would like to develop into a candidate, they will offer you either prep school or civil prep (i.e. Marion Military, etc.). However, if you've applied and gotten the thin white envelope, I believe your best course of action is to go to college, enroll in ROTC, and excel. Make amazing grades and work your arse off in ROTC. Reapply as soon as the application opens and complete it early. 25% of each class does not get in on their first attempt. If you really want this as you say, try again.