Aviary Hall Series by Penelope Farmer

A strange, magical boy approaches Charlotte and Emma Makepeace on their way to school one day, and offers to teach them to fly. Soon all of the children in the village have have learned, and are given one precious summer of flight and freedom, although they must struggle to keep their outings secret.

Lonely and unhappy after her sister goes away to school, Emma is amazed when she discovers that she and her awkward classmate Bobby Fumpkins have something in common--they are both being propelled backwards in time while they sleep . . . and becoming friends.

After Charlotte's first night at boarding school, the view from the window has changed and the girl in the next bed is not the girl who was there last night. She is a stranger who calls Charlotte "Clare" and says she's her sister...