Loudmouth Louis by Anne Fine
Everyone knows that Louis is a loudmouth, he never stops talking and drives everyone crazy. No one believes him when he announces that he's going to do a sponsored silence to raise money for the new school library. But Louis is determined, just for once, to keep his mouth shut. To his surprise he finds that he actually enjoys listening, that lessons are much better when he's really involved in them - and that it's very satisfying to raise money for the school.