Nicholas is a creative, red-headed fifth grader who loves to invent harmless but disruptive ideas. He previously turned his third-grade classroom into a tropical beach and mimicking bird calls in fourth grade. He is highly intelligent and perceptive, often using his quick thinking to delay homework with strategic "thought-grenade" questions. His lively imagination thrives on making school more engaging for his peers, though he rarely anticipates the broader consequences his schemes create.

Mrs. Granger is an older, sharp-eyed educator known for her unwavering dedication to vocabulary and traditional discipline. She maintains a formal classroom environment, famously wearing skirts and jackets regardless of the physical temperature. Her intense gaze and strict weekly dictionary assignments earn her a severe reputation among students, though she genuinely cares about developing their intellect and critical thinking skills.

Ginny is Nicholas's mother, serving as his strongest advocate and moral stabilizer. She values her son's creativity and is unafraid to challenge authority figures when she feels a reprimand is an overreaction to a harmless joke. At the same time, she enforces household rules strictly and expects Nicholas to show genuine respect toward his teachers.

Tom is Nicholas's father, a mild-mannered man who prefers a quiet life and actively avoids unnecessary fuss. He feels completely out of his element when his son's actions generate local attention, and he attempts to placate upset authority figures rather than fight them. He tries to guide his son by advising him to consider how his practical jokes might negatively impact others.

Mrs. Chatham is the principal of Lincoln Elementary School. She views her role as maintaining order and tradition, putting her directly at odds with the students' disruptive new vocabulary game. While she projects immense authority to the children, she becomes visibly uneasy when pressured by parents, the school superintendent, or the local press.

Judy is a persistent reporter for The Westfield Gazette who typically covers mundane local events like tree plantings and minor burglaries. Hungry for a more engaging story, she aggressively pursues the drama unfolding at the elementary school. She writes sensationalized articles that amplify the tension between the students and the school administration.

Janet is a classmate who occasionally participates in Nicholas's classroom schemes. She walks home with him after school and inadvertently sparks a massive new idea when she finds a gold writing utensil on the side of the road.

Bud is a lifelong Westfield resident with a sharp entrepreneurial spirit. He recognizes a profitable trend and moves quickly to capitalize on the town's latest craze, eventually seeking legal protection and a formal arrangement to secure his business venture.

Alice is a television news reporter who recognizes the broader appeal of the elementary school dispute. Backed by major networks, she secures on-camera interviews with key figures in town to broadcast the story to a national audience.

Miss Deaver is a naive, first-year third-grade teacher. She is easily charmed by her students' creativity, allowing their harmless ideas to escalate into complete disarray before the administration steps in to stop the fun.

Mrs. Avery commands an orderly fourth-grade classroom. She is strict but fair, demonstrating a willingness to apologize directly to a student when she realizes she has wrongfully accused them of causing a disruption.

James is Nicholas's older brother, currently attending college. His academic success and realization that studying matters serves as the family's concrete proof that a rigorous "homework first" policy produces excellent results.

John is a fifth-grade classmate who actively participates in the vocabulary rebellion. He helps execute classroom stunts that exasperate their teachers, proving to be a reliable accomplice.

Chris is another of Nicholas's close friends and classmates who participates in the daily life and schemes of the fifth-grade class.