Here are the rules for becoming a higher ranking ranking officer:


First: To get higher ranking you must pass the kidou and basic shinigami academy with proficient or higher standards viewed by the captain-commander or a captain.
Second: Once passed with the academy you will be placed in one of the barracks to train until regarded as a squad member by a captain.
Third: If a captain has viewed your background status with normal results and no felony's towards Soul Society or outside of Soul Society.
Fourth: Once all is clear you will be given a trial in which a squads captain will interview your skills and grades you on your kidou, combat, and behavior.
Fifth: Once the captain has accepted you into their squad then you must begin training outside of soul society against weak hollows or train with your Zanpakutou.
Sixth: When a captain views your progress and decides your doing well its up to you to show you captain even greater skills. And then the captain decides if you can rank up or stay at your rank.