Looking for a rare science fiction book from when I was in Junior High.
We read this close to the same time we read the Chrysalids and Lord of the Flies. This book was about a girl and a boy that became friends, but they lived in a different world. The girl treated the boy as though he was weird, because he looked different and he lived at the bottom of the mountains in the grasslands. Close to the end of the book the girl comes down from the mountains or comes across a mirror and realizes she's the weird one after all - she is designed to survive up there. She has thick blue skin, large nostrils to take in the thin air, etc., etc.
I can't remember the author or the title, but I remember the female character describing how she came down to where the boy lived and the air was thick as soup for her to breathe.