There's a Boy in the Girls' Bathroom - Louis Sachar

Bradley Chalkers is the oldest kid in the fifth grade. He tells enormous lies. He picks fights with girls. No one likes him—except Carla, the new school counselor. She thinks Bradley is sensitive and generous, and knows that Bradley could change, if only he weren't afraid to try. But when you feel like the most-hated kid in the whole school, believing in yourself can be the hardest thing in the world...