Aviation theory for fighter pilots
My experience is with US students who have joined the US Air Force, Navy or become Army Aviators.

My non-flight study guide includes:


Critical thinking
Physics problem solving
Math problem solving
argumentation (related to critical thinking)
FAA Private Pilot and Instrument Pilot training material
where possible visits to towers, TRACON, etc.
volunteering at:  glider club, medvac company
public speaking
STEM college degree, not necessarily in aeronautical science


In college there will be more opportunities, and additional guidance.  Many high school students have learned to fly.  It's nice to have former students who are flying F-16s in various parts of the world.  Some who wanted to be fighter pilots ended up doing things like B-2s, tankers, etc.

Since most pilot jobs involve being an officer, leadership skills are important, and help differentiate applicants. You will always have to somehow differentiate yourself, to be competitive. Every former fighter pilot I know is very competitive.

Best.