If you attend any of the service academies this is how it works: You can voluntarily leave at any time up to the first day of classes your junior year with no obligation. If you leave after that, regardless of the reason (dismissed for grades, honor violation, you just quit, etc.), you are committed to serve as an enlisted person in the AF for a period of time (normally two years on active duty, possibly some reserve time). Once you graduate, you are commissioned as an officer in the active AF and have a five-year commitment following graduation. If you become a pilot, you must serve ten years following the completion of pilot training primarily because it costs between $1.5 and $2M to train a pilot.
The most famous professional athletes who graduated from the service academies are men such as Roger Stauback who served in the Navy and came back to a Hall of Fame career in the NFL; Chad Henning (AF A-10 pilot) who served and came back to be an all-pro defensive tackle for the Dallas Cowboys; and David Robimson who served on both active duty and the reserves before becoming a Hall of Fame NBA player for the Spurs. The rules are slightly different today with waivers given over the years to play professional sports after serving in some capacity -- depending on the particular circumstances and who was serving as President at the time. Supposedly, President Trump is considering changing some of the policies to be a little more lenient, but as I said, it depends on who is President and what they want to allow through policy change. Hope this helps. My class at the AF Academy started with just over 1400 freshmen and graduated around 700. So, a lot of attrition occurs naturally and varies greatly over the years and depending on world events. I swerved 30 years and retired as an AF General.