I read a book and its sequel in the mid 90s, but they may have been published earlier. For some reason I think it was the late 70s. I checked them out at my middle school library, so they were juvenile or young adult fiction. The first book was about a society of fairies or maybe just really small people that live in the tops of trees. The young people are assigned a vocation based on their abilities when they are in their early teens. They start out as novices. I think the main character was a girl, but she had a really close guy friend. In the 2nd book, the protagonist discovers another similar society underground in the root system at the base of the tree. Not much to go on, but can anybody help identify?
Your question has elements of the Green Sky Trilogy by Zilpha Snyder.
From the linked Wikipedia page:
The planet Green-sky contains Orbora, a city built in giant trees and inhabited by humanoids known as the Kindar. They have built their homes among the branches, and can harvest food from vines and orchards found on the trees. The Kindar walk between trees where the branches overlap or connect using vines woven together like ladders. Low gravity also allows Kindar to fall great distances without being hurt and hang-glide through the air using their shuba, a silk cape-like garment that attaches at hands and feet.
Entire society lives in the trees.
I don't see any mention of just living in treetops, but they are forbidden to touch the ground, that appears to be a close fit.
Not fairies, but gliding and near-flight can give that impression.
Exiles have indeed started their own society underneath the ground and below the roots of the trees.
At age 13 young ones are chosen for their position in the society by the priests called Chosen, and those with strong psychic powers ("spirit force") are chosen to join the priests.
Published between 1975 and '78, so matches your guesstimate.
Main character is Raamo, a boy who's joining the Chosen, but there is mention of his sister and a fellow recruit who's a girl.