Rachel is a quiet sixth-grade student who loves reading and dreams of becoming an airplane pilot. She has diagnosed selective mutism, choosing to remain completely silent following the sudden death of her classmate Tommy six months prior. Using written notes and hand gestures interpreted by her best friend, she navigates the school day while privately carrying a deep sense of guilt over how she treated Tommy before he died.

Bastian is an "Air Force brat" facing his eighth family relocation in twelve years, this time to Hawaii. He uses a brash, bullying exterior to keep his peers at a distance, preferring not to form attachments since he always has to leave. His primary concern is his energetic puppy, Barkley, who faces a mandatory four-month quarantine upon moving.

Karen is the responsible and capable class president of Room 238. When the substitute teacher fails to arrive, she becomes the de facto leader, guiding the class through their daily schedule. She views the unsupervised day not as a chance to misbehave, but as a genuine test of the students' character and maturity.

Jessica is a tall, strict student who dreams of becoming the Chief Justice of the Supreme Court. Raised by a lawyer father, she views the world through a rigid legal lens. She is the solitary voice of opposition when the class decides to run itself, constantly worrying about the legal liabilities and rule-breaking involved.

Sean is a quiet boy from a low-income, neglectful household who regularly fends for himself. He struggles academically, particularly with reading, and receives extra help in the Resource Room. He finds peace in solitary walks through the woods and harbors a gentle crush on Rachel, completely unfazed by her mutism.

Missy is Rachel's highly empathetic and supportive best friend. She serves as Rachel's voice in the classroom, reading her written notes aloud and interpreting her facial expressions. Missy never pressures Rachel to speak, providing a safe and accepting presence that anchors Rachel throughout the school day.

Tommy is a deceased sixth-grade student who died of natural causes exactly six months before the events of the novel. He was older than his classmates, having been held back twice. He had a massive crush on Rachel and frequently brought her gifts, including baked goods from his family's bakery, which she harshly rejected the day before he died.

Mr. Fabiano is the popular, charming teacher of Room 238. His unexpected absence creates the central situation of the novel. He places a heavy emphasis on building good character in his students, establishing an environment of trust and responsibility that continues to influence the class even when he is not physically present.

John is Bastian's best friend. He initially plays hooky on Bastian's last day but later joins the unsupervised classroom at Bastian's urging, participating in the class's secret rebellion.

Barkley is Bastian's energetic and beloved puppy. Because the Fauvell family is moving to Hawaii, Barkley faces a mandatory four-month quarantine, a situation that causes Bastian significant distress throughout the day.

Darlene is the twenty-two-year-old girlfriend of Sean's father. Living in a chaotic and neglectful household, she attempts to provide some minimal assistance to Sean, such as offering to help with his laundry.

Mr. Peacock is the principal of Paulson Elementary School. He remains entirely unaware that Mr. Fabiano's class is functioning without an adult, occupied instead with administrative complaints and organizing a school assembly.

Mrs. Muchmore is the substitute teacher originally assigned to cover Mr. Fabiano's class. Her sudden illness and a subsequent miscommunication in the main office directly lead to the students being left unsupervised.

Helen is the busy school secretary who receives Mrs. Muchmore's sick call. Due to a messy distraction in the principal's office, she accidentally loses the message and forgets to secure a replacement substitute.

Sky is a new student in Room 238. When he suffers a bleeding injury at recess, his need for medical attention threatens to expose the class's secret, forcing the students to handle the first aid themselves.

Monica is Jessica's three-year-old little sister. Her messy attempt to assert independence at breakfast results in spilled juice and a tense argument between Jessica and their father.

Mr. Snickenberger is the school psychologist who attempted to treat Rachel's selective mutism. He eventually lost his patience with her silence, harshly telling her she cannot run away from her life.