The Eighth Room - A Beth-Hill Novel: Jacob Lane, #3

Jacob Lane is a ten-year-old girl who's spent her life unaware of her magical heritage. After being sent to Darkbrook, a school of magic, supernatural mysteries seem to spring to life all around her and her new friends. For two hundred years, the Selkies have kept themselves separate from those who live on land. But now the Selkies need allies or they'll be crushed by their ancient enemies, the Finfolk.
Jacob and Ophelia, students at the only school of magic in the United States, uncover a mystery that dates back to Darkbrook's beginnings. While helping clean out old storage rooms for classroom expansion, they find something that might save the Selkies from extinction. With the help of the youngest member of the Wild Hunt who are no longer so wild or terrifying, they must foil the Finfolk who desire the Selkie's destruction...or die trying.