To expand a bit more, the three DOD service academies, USMA, USNA, USAFA have their own prep schools. They offer seats there to applicants they like but need some more preparation work before tackling the SA curriculum. These 3 academies also have scholarship programs for well-known college prep schools which have service academy prep programs. Same deal, the SA chooses who they want to offer a scholarship to. You can also “self-prep” at these schools, paying your own way, as a path to application to an SA. USCGA and USMMA also have some programs.
If you are just starting to think about a service academy, the best thing you can do right now is go to their sa.edu website, and read every page, hamburger menu item and link. You will find profiles for classes, recommendations for HS courses to take, etc.
Here’s an example:
General Advice for Grades 9-12 page for Admissions at USNA.edu. Updated Fri Apr 12 05:31:35 EDT 2024.
www.usna.edu