What does a Military Academy do?

Im going to Vally Forge Military Academy for my whole high school,but thats not the point.. I heard that if you join a Military academy and serve some years, you'll become a Commanding Officer,is that true? when i join the Navy, Will i be a Commanding Officer? And if i move i'm going to Military Academy in Guatemala (maybe) if i join the US Navy, will i still be a Commanding Officer in the US Navy? what does a Military Academy Truly do?