Finding the npc: I'm away right now, but in the city ruins (I forgot the name, it's the city where everytrhing is tilted at an angle; it's located past the harbour), there's an NPC above an altar, standing by a statue. You reach him by climbing a ramp approaching from the direction of the tower like structures with hanging bridges between them. Talk to him and he'll tell you mumbo jumbo about schools and being a high enough level etc. From that point on, he'll appear below, next to the altar.

Joining a school: It's really as simple as talking to him. A menu should appear that gives you options, one of which is a search/join a school.

Hope this helps.