Every Heart a Doorway by Seanan McGuire

Eleanor West's Home for Wayward Children: No Solicitations. No Visitors. No Quests.

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. 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 things.