The Other Girl - Emily Barr, Penguin Random House Children's UK

Two girls meet on a train journey. One of them - privileged, chaotic - is heading to an exclusive Swiss rehab centre following months of partying and living wildly. The other - who couldn't be more different - is down to her last penny, running away.When they get off the train they are both pretending to be the other. Just for a short period, of course. Just until the rehab stint is over .