Unschooled — Allan Woodrow

This year's fifth graders are the worst Principal Klein has ever seen. But he's hoping that Spirit Week can teach them teamwork, with a top secret prize for the winning team as incentive.

Best friends George and Lilly have been looking forward to Spirit Week all year. They might be complete opposites, but they can't wait to be on the winning team together. When their classes end up rivals, with Lilly leading Team Red and George leading Team Blue, the friends swear they can compete and remain best friends.
But suddenly there are slimed lockers, sabotaged costumes, and class pets held hostage. As the pranks escalate, it threatens everything, including the prize. Because if Principal Klein finds out, Spirit Week will be cancelled and the students will spend the rest of the year in detention.

Can George and Lilly find a way to fix their friendship and get the entire fifth grade to play fair, or is the most awesome week of fifth grade about to make this the worst school year ever?