The Astonishing Chronicles of Oscar from Elsewhere by Jaclyn Moriarty

Oscar is just skipping school and hanging out at the local skatepark when he suddenly finds himself transported into a world very different to his. And this is his own account of his adventures from Monday to Friday last week. In the company of Bronte Mettlestone, Esther, Imogen and Alejandro, ordinary Oscar finds himself on a quest to locate nine separate pieces of a key, held by nine separate people, in order to unlock a complicated spell that had trapped the Elven city of Dun-sorey-lo-vay-lo-hey.