Schools in Meridian59 represent the abilities you can use to adventure in the world.
Each school is based on certain levels of advantage. You'll advance into the next level whenever you're trained enough in the skills of the current level.
To join a school you need to buy at least one level 1 ability off a school trainer. Each character automatically gets the spell blink on creation, which is Riija level 1. This spell does not count in so you're not a student of Riija unless you buy another spell of the school.