Out of Time — Caroline B. Cooney

Annie Lockwood exists; everyone admits it. Everyone has seen her. But only Strat insists that Miss Lockwood traveled one hundred years back in time to be with them in 1895. Now Strat is paying an enormous price: His father has declared him insane and had him locked away in an asylum. When Time calls Annie back to save Strat, she does not hesitate, even though her family is falling apart and desperately needs her. Can Annie save the boy she loves, or will her choice keep her a trespasser out of time?