The Mysterious Benedict Society  (English, Paperback, Stewart Trenton Lee)

When a peculiar advertisement appears in the newspaper for children to take part in a secret mission, children everywhere sit a series of mysterious tests. In the end, just four children succeed: Reynie, Kate, Sticky and Constance. They have three things in common: they are all honest, all remarkably talented and all orphans. They must go undercover at the Learning Institute for the Very Enlightened where the only rule is that there are no rules. There they must work as a team to save not only themselves, but also the world outside the walls.