Marking Time by April White
Summary: Seventeen-year-old tagger Saira Elian can handle anything … a mother who mysteriously disappears, a stranger who stalks her around London, and even the noble English grandmother who kicked Saira and her mother out of the family. But when an old graffiti tag in a Tube station transports Saira to the nineteenth century and she comes face-to-face with Jack the Ripper, she realizes she needs help after all.
Saira meets Archer, a charming student who helps her blend in as much as a tall, modern American teen can in Victorian England. He reveals the existence of the Immortals: Time, Nature, Fate, War, and Death, and explains to Saira that it is possible to move between centuries – if you are a Descendant of Time.
Saira finds unexpected friendships at a boarding school for Immortal Descendants and a complicated love with a young man from the past. But time is running out for her mother, and to save her, Saira must embrace her new identity as she hides from Archer a devastating secret about his future that may cost him his life.