Many students at MSF High were once military members of one of the main factions. While there are smaller military groups that have different ranks, the most standard ones for the various factions are as follows:
Golden Alliance
Golden Alliance ranks are based loosely on standard Altearthean military ranks, with naval ranks adapted for space ranks. Higher ranks mean higher pay and more responsibility, but also more seperation from combat in most situations.
Castaways Treaty Systems
CTS ranks are based on a simplified GA ranking scheme, with "alpha" versions of each rank acting as a lead for a group of 3-5 of their peers. The special Warpmage ranks are seen as advisors with no specific rank-based powers but large amounts of influence when dealing with matters of their specific fields.
Angelic Protectorate
AP ranks are based purely on one's experience and prestige in honorable combat. Higher-ranked Divine are given a higher priority for position in a battlefield as well as better support and the right to command others below them. When a Divine reincarnates, they usually drop a rank or two, although divine are often given special additional title prefixes such as "grand" or "master" that they retain even after reincarnation.
Elemental League
EL military members have very little difference in pay grade, merely in responsibility, and as one proves themself able to handle more responsibility their number suffix is increased. Servicemen are generally technical specialists, while volunteers are frontline soldiers and leaders are obvious. A rank IV volunteer may be considered more meritorious than a Serviceman V or even VI, depending on the individual.
Legion
Legion generally don't have actual ranks, but ones who are assigned to head tasks are called "commanders" to outside observers and partners of other races. Legion refer to each other as "sister" most of the time, though they tend to refer to their specific queen as "mother" and are referred to by her as "daughter". Princesses, while not enjoying any benefits of command, handle duties that allieviate the Queen's own and are promoted genetically. While Queens generally respect each other as equals, all of them would gladly obey any advice from the First Queen, Althea, were she still alive.
Galactic ANarchists
No official ranks, though various captains may refer to themselves by any rank or self-aggandizing made-up title, command preference is determined based on who's got the largest number of troops at their disposal.
Watchers
Watchers that arrive at MSF High have all been Students, and while the exact nature of their command structure isn't known, this is about the best that they've got.