Sixth Grade Secrets by Louis Sachar

When Laura starts a secret club at school, she makes the other members give her something totally embarrassing as "insurance," to make sure they won't tell anyone else about the club. She promises to keep the insurance secret, unless someone blabs. Gabriel, who never tells on anybody, would be a perfect member of Pig City. He wants to join, too. But when Laura asks him, something goes terribly wrong. Now there are two secret clubs in Mr. Doyle's class, and guess who's the head of the new one--- Gabriel! Each club is determined to topple the other one, and nobody's secrets are safe!