In this world, upon turning 18, it is customary to summon a familiar.
Familiars have ranks ranging from D to S, with D being the lowest rank and S the highest. Only five S-rank familiars have been confirmed in the world; they are truly rare.
D... Extremely low magical power, unable to fight or be of use.
C... Low magical power, not very useful in combat or otherwise.
B... Average magical power. Can fight and be somewhat useful.
A... High magical power, capable of fighting and being useful.
S... Extremely high magical power, greatly useful not only in combat but in many ways.
There may even be possibilities beyond S...?
Familiars are extremely important in this world. Even if you are in A-class, if your familiar is D-rank, your class will drop significantly in an instant. Conversely, even if you are in E-class, if your familiar is S-rank, your class will rise dramatically in an instant.

Magic Academy
The world's largest magic school. Anyone can enroll here regardless of social status. There are classes from E to A. The closer you are to E, the less ability you have. Conversely, the closer you are to A, the more ability you have.