Miles is a middle-school student who recently moved to the rural, cow-obsessed town of Yawnee Valley. He carries a detailed notebook filled with diagrams and instructions for his past pranks. He hopes to establish himself as the supreme trickster at Yawnee Valley Science and Letters Academy to avoid being labeled just another new kid.

Niles presents himself as the ultimate teacher's pet, wearing a sash that reads "School Helper" and maintaining a pristine public image. Beneath this angelic exterior, he possesses a sharp, calculating intellect and a vast knowledge of historical mischief. He operates a secret planning room in his closet and views trickery as a highly structured art form.

Principal Barkin is the authoritarian leader of Yawnee Valley Science and Letters Academy. He comes from a long line of Barkins who served as class presidents before becoming school principals. He loves order, protects his yellow car fiercely, and obsesses over maintaining an unbroken family record of keeping the school open.

Josh is the school bully and the perpetually sitting class president. He leverages his father's position as principal to intimidate other students and escape consequences for his physical aggression. He maintains his political power by rigging the annual elections.

Holly is a perceptive, sarcastic student at Yawnee Valley Science and Letters Academy. She refuses to be intimidated by the school's corrupt student council politics. She runs for class president every year on a platform of honesty, knowing perfectly well that the election process is rigged against her.

Judy is Miles's mother, who recently moved the family away from their beloved home near the ocean. She works at the local post office and tries to stay optimistic about their new life in a rural, cow-filled town. She worries constantly about her son's adjustment to his unfamiliar environment.

Ms. Shandy is a teacher at Yawnee Valley Science and Letters Academy. She attempts to maintain order and deliver her lessons despite the frequent disruptions caused by student hijinks. She assigns oral reports and oversees daily class activities under the watchful eye of the principal.

Stuart is an easily amused student at Yawnee Valley Science and Letters Academy. He frequently laughs out loud, even when his classmates do not find the situation funny. His gullible nature occasionally makes him an easy target for trickery.

Bob is Principal Barkin's brother. He works at the local junkyard and manages a dairy farm. He operates heavy machinery, handling the physical labor that his brother sometimes requires to resolve unusual school emergencies.

Bertrand is the former principal of the school and the demanding patriarch of the Barkin family. He places immense pressure on his descendants to maintain their absolute authority over the student body. He values the family's unbroken record of keeping the school open above all else.

Grandpa Jimmy is a deceased past principal in the Barkin family lineage. He holds a disgraced place in the family history because he once closed the school during the historic blizzard of 1932. Despite his reputation among the older Barkins, he had a good heart and a slight sense of humor.

Mrs. Conlon is a cafeteria worker at Yawnee Valley Science and Letters Academy. She manages the lunch line and interacts with the students as they receive their food.