Active duty members applying to the academy, are looked at totally differently. I won't go into the details; but I will say that what an enlisted person does in the application process is not necessarily done or looked at the same.
As for applying to the academy for a high school student, the best preparation you can do, is to ensure you are taking the most challenging academics that you are capable of excelling in. Also; be involved in athletics and other activities that promote team work and leadership experience. Work out as much as possible with running, doing pullups, and situps. You do that, and you'll be fine.
I always recommend that individuals get their application completed as soon as possible. That includes the CFA. Even the application paperwork "Suggests" trying to complete it in 45 days. "Yes, it it very possible". The only individuals I recommend hold out on the CFA, and those their application, are those who have an "Average" application. This way they can try and get "More Points" in the CFA. But the individual who is the 3.9+ gpa; kick butt SAT/ACT, plays and letters in varsity sports, is involved with a lot of volunteer and leadership activities, etc... Get your application done as soon as possible. Even with an "Average" CFA. Remember: Until your application is 100% COMPLETE: minus a nomination and your DODMRB physical, the academy will NOT EVEN LOOK AT IT. So for those with an outstanding application; getting it submitted early is a benefit. You will be adding new ACT/SAT scores and accomplishments to it, and it will be looked at more often. Especially for an early LOA or appointment if you have a military related / non-moc related nomination. Best of luck. mike....