The mysterious Benedict Society - Trenton Lee Stewart

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.