The Sixth Form at St Clare's recap

Pat and Isabel begin the term as joint head girls, with the Sixth Form charged to advise younger pupils. Dora's baffling behaviour is explained when her hidden twin Daphne is discovered. Carlotta helps the hostile newcomer Morag face the grief and resentment behind her rejection of school, while Miss Theobald secures Carlotta's own educational future. The advice scheme is threatened when Priscilla misuses private disclosures, blackmails Joan over her father's embezzlement case, and forces her to steal. Priscilla is expelled, Joan is supported back into her form, and another man's confession clears Joan's father. Angela and Mirabel pass their repeat examinations. At term's end Fizz reveals that she is Lady Phyllis Bentley, Claudine accepts an invitation to join her family for the holidays, and Hilary leaves St Clare's after six years.

Previously: Pat, Isabel, Alison, Hilary, Bobby, Carlotta, Gladys, Janet, Claudine, Angela, and Mirabel are established pupils at St Clare's. Miss Theobald is the school's headmistress. They enter this term with existing friendships and school experience, ready to take on the greater responsibilities of the Sixth Form.

Pat and Isabel return as joint head girls. Carlotta learns that her father wants finishing school rather than university for her, while Angela and Mirabel prepare to repeat their examinations. The hostile newcomer Morag Stuart is placed with Carlotta, and Miss Theobald gives the Sixth Form weekly meetings at which younger pupils can seek advice. Gladys becomes games captain, with Bobby and Janet assisting her. New pupil Fizz Bentley makes a favourable impression on most of the form. Meanwhile, First Former Dora Lacey seems able to cross the school impossibly fast and sometimes forgets skills she had just displayed. Anne-Marie finally sees Dora enter a shop twice without leaving. Pat, Isabel, and Anne-Marie confront two identical girls and discover Daphne, Dora's twin. The sisters admit that Daphne has been hidden at school while they alternated between lessons, meals, and one dormitory place. The head girls decide that Miss Theobald must be told.

Miss Theobald punishes Dora and Daphne for their deception and requires them to confess to Miss Roberts. The twins remain admired by many First Formers, but timid Joan Terry becomes increasingly isolated and dependent on Alison's approval while Priscilla intimidates her. The Sixth Form's first advice meeting addresses Hilda's fear of confronting Ruth over stolen ideas and another pupil's misuse of Susan's lost money. Priscilla favours punishment, unlike the others' attempt to repair the problems. Morag's rudeness and poor work intensify her conflict with Pat. Carlotta first frightens her into working by claiming she could be moved down a form, then discovers their shared love of riding. After another quarrel, Morag explains that her mother's death, her father's remarriage, and the loss of her former freedom made her resent home and school. Carlotta draws on her own difficult adjustment to help Morag reconsider. Morag resolves to change, though she has not yet repaired matters with Pat.

The First Form plans a midnight party for Rita's birthday. Priscilla overhears enough to interfere and uses private information from the advice meetings to provoke quarrels between Susan and Lucy and between Ruth and Hilda. Both disputes are resolved, but the younger pupils recognize the breach of confidence and boycott the next meeting. Morag apologizes to Pat and the Sixth Form, crediting Carlotta with changing her outlook. Katie and Hilda warn the head girls that Priscilla is seeking the party details, so the First Form prepares a false date for her to discover. Priscilla forces Joan to spy and receives the false Wednesday plan, while the real party succeeds on Tuesday. The next night Priscilla follows staged evidence and reports a nonexistent party. Dora throws Priscilla's missing slipper against Mam'zelle's door, making the accuser appear responsible for the disturbance. Mam'zelle and Miss Harry find no gathering, and Priscilla is ordered to face Miss Theobald.

Miss Theobald reprimands Priscilla and recommends excluding her from the advice meetings. In revenge, Priscilla forces Joan to steal from Sixth Form pupils, using the threat of exposing the embezzlement charge against Joan's imprisoned father. Alison and Bobby find the missing property in Joan's locker, but Joan explains the coercion and insists on her father's innocence. Pat, Isabel, and Alison take the case to Miss Theobald. Priscilla is expelled, while Joan receives a grave but sympathetic warning. Dora and Daphne are asked to draw Joan into First Form life, and their support restores her standing. Another man then confesses, clearing Joan's father. Angela and Mirabel obtain the examination results they need, and Miss Theobald persuades Carlotta's father not to send her to finishing school. At the close of term, the Sixth Form holds an approved farewell party for Hilary. Fizz reveals that she is Lady Phyllis Bentley, whose title was concealed with Miss Theobald's consent, and invites Claudine home for the holidays. Hilary leaves after six years at St Clare's.

Pat and Isabel remain joint head girls after a term that tests the Sixth Form's new responsibility for younger pupils. Priscilla is expelled for blackmailing Joan and forcing her to steal. Joan receives a serious warning but is protected as a victim of coercion, welcomed back by her form with Dora and Daphne's help, and reunited with her family after another man confesses and clears her father. Morag has reconciled with the Sixth Form and intends to improve matters at home. Miss Theobald persuades Carlotta's father to abandon the finishing-school plan. Angela and Mirabel pass their repeat examinations, and Angela can proceed to finishing school. Fizz discloses that she is Lady Phyllis Bentley and invites Claudine to spend the holidays with her family. The Sixth Form gives Hilary a farewell party before she leaves St Clare's after six years, while the other major pupils expect to return after the holidays.