And... it seems that the assumption is correct. In A Time of War, Military Academy section of the character creation says that;
.. As the place where MechWarriors learn their craft and all recruits are viewed as potential officer candidates, military academies have stricter entry requirements than simple enlistment.
And, although it is not forced, but Military School in A Time of War is divided to three groups;
-Academy: Provides advanced curriculum. You can learn...
--Advanced: Cavalry(Combat Vehicle), Infantry, Marine, MechWarrior, Aerospace/Conventional/Dropship Pilot, Scientist, Scout, Ship Crew, and analysis.
--Special: Doctor, Anti-Mech(infantry), Military Science, Battle Armor Pilot, Jumpship Pilot, Special Force.
-Enlisted: It is a lesser school that is focus on the conventional force. You can learn...
--Advanced: Cavalry, Infantry, Marine, Medical assistant, Police officer, Scout, Ship Crew, Military Technician.
--Special: Police Tactical Officer, Anti-Mech(infantry), Special Force, Vehicle/Aerospace Technician.
-House: Trained in the individual noble house rather than the normal military school.
You can learn basic(not on the list I have shown), then can also learn either advance or special, or all basic, advance and special.
And, there is an another optional school named Officer Candidate School, that only accepts the person graduated military, paramilitary, or police school. It takes one year of training and and it allows the officer rank to the character. The basic military training above doesn't give the officer at all, and you must also pick the OCS as well(you can pick multiple schools as long as you want to pay for the years).
So, it seems that the 'academy' doesn't gives all the graduates officer ranks, and the term academy in Battletech only means the 'advanced military training school' rather than what we think of the word 'academy' - the military school that make the officers. Although some graduates starts their career with the lowest ranked officer, but they are just have the OCS curriculum on their school.
Yes, it is only the character creation mechanism on A Time of War, but I am surprised that the word 'academy' doesn't make them officer.
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edit: I found that it was originally SLDF's tradition that the graduates of the academy doesn't commission immediately and have sergeant instead, and after they prove themselves they are commissioned.
...And, I also found that Taurian aerospace pilot start their career as force sergeant which is enlisted rank, rather than officer. Wow, I didn't aware that there are any major nations that have enlisted pilots. Although they are also promoted to ensign same as most nation's mechwarriors.