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On the inaugural day of high school, Ayako, a 35-year-old, makes her debut as a student. Unfazed by the peculiar school hierarchy and the relentless bullying, she boldly tackles every issue that plagues students.
Ayako shines academically in her class. However, her innocent question inadvertently exposes the fact that Rina always eats her lunch in the bathroom, causing Rina to be absent from school the following day.
Ayako's fellow classmates hold animosity toward her and manipulate Ai into harassing Ayako. In her role as the class representative, Ayako discovers a note in the suggestion box with the desperate appeal, "Save me."
Ayako is disgusted with her homeroom teacher, Koizumi for segregating particular students by school castes. Later, she finds a website that unveils the pervasive influence of the school caste system throughout the entire school.
The school festival nears, and Rikako steps into the role of leader. Preparations go smoothly at first, but neither Rikako nor her friends make any effort. Tensions boil over between Rikako and Ayako, culminating in Rikako storming out of the classroom.
Just before the important Ekiden race, Moe suffers an ankle injury. Eto, who took Moe's place, falls ill mid-race, resulting in their team's withdrawal. Mitsuki, who was eyeing a sports referral, directs her anger towards Eto.
In a mock teen court, Koizumi's class addresses Mitsuki's accusation of Yuna taking unauthorized photos of her. The class chooses voyeurism as the topic, and Mitsuki picks Ayako as her defense attorney.
Koizumi escorts Eto to her house as she's being followed. The next day, a photo of them surfaces on the school's website, causing a big commotion. Observing Koizumi's visible anger, Ayako senses that he's concealing something from his past.
To make the students reevaluate themselves, they are sent to a three-day survival camp. They also adopt a vote-based system for eliminating undesirable individuals, with Ayako being the first to face expulsion in the initial vote.
The class begins to shed its hierarchical divisions, leading to a more cheerful atmosphere. In the midst of this, a rumor starts to circulate that Ayako is attending high school under Asada's instructions, participating in secretive experiments.