The Name of the Star by Maureen Johnson

Louisiana teenager Rory Deveaux arrives in London to start a new life at boarding school just as a series of brutal murders mimicking the horrific Jack the Ripper killing spree of more than a century ago has broken out across the city. The police are left with few leads and no witnesses. Except one. Rory spotted the man believed to be the prime suspect. But she is the only one who saw him - the only one who can see him. And now Rory has become his next target...unless she can tap her previously unknown abilities to turn the tables.