What is your deadline to accept the Prep School offer- is it May 1? Technically, Yes, you can accept the USAFA Prep offer, then withdraw your acceptance, to accept a direct offer to USNA, but if your deadline to accept USAFA Prep isn't right around the corner, I would hold off on accepting until you hear from USNA.
Think through your decisions thoroughly and listen to your heart. There is no rule against changing your mind after accepting an offer. A personal moral issue may arise when and if multiples offers are accepted for the safety/backup purpose due to offer recipient's indecisiveness.
Do pay attention to the different deadlines of the different offer sources should/when you get more than one. Once an offer is declined, there's not a second chance given by that source. It's a good feeling if you get multiple offers. There's always the "what if" scenerio.