Yes, everyone is in school all 4 years, but not all will take the same semester course loads. Showing up with AP/IB/college courses can make your life easier.
Here are some fun facts from the Course of Instruction Handbook:
1) USAFA requires at least 125 hours of in-residence credits.
2) All cadets will take 10 credits of physical education courses.
2) Each semester has a minimum credit-hour requirement. Meeting the minimums every semester equates to 134 credits total (including 10 phys ed credits)
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3) Most majors require 140-145 credits, but some as many as 150 (Chemistry Engineering track).
4) Course load waivers are available, but generally for people seeking to take over the maximum, not under the minimum.
My main point is that a cadet who has substantial AP/IB/previous college could consistently have semesters with 4-5 classes as opposed to the normal 5-6 classes. Cadet life with 1-2 fewer academic courses per semester means that you can focus on other things like clubs, sports, flying, or taking naps.