Every Heart a Doorway by Seanan McGuire
Synopsis from GoodReads:
Eleanor West’s Home for Wayward Children
Children have always disappeared under the right conditions; slipping through the shadows under a bed or at the back of a wardrobe, tumbling down rabbit holes and into old wells, and emerging somewhere… else.
But magical lands have little need for used-up miracle children.
Nancy tumbled once, but now she’s back. The things she’s experienced… they change a person. The children under Miss West’s care understand all too well. And each of them is seeking a way back to their own fantasy world.
But Nancy’s arrival marks a change at the Home. There’s a darkness just around each corner, and when tragedy strikes, it’s up to Nancy and her new-found schoolmates to get to the heart of the matter.
No matter the cost.