Mr. Thackeray (Sidney Poitier) is beginning his teaching position at a tough London high school. He comes from British Guiana via the United States, and holds a degree in Communication Engineering. He has applied unsuccessfully for numerous engineering positions, and is taking this job to make ends meet while he continues to look for a job in his field.
The staff offers varying opinions about students at this tough school. Mr. Weston (Geoffrey Bayldon) is openly contemptuous of them. Gillian Blanchard (Suzy Kendall) is admittedly afraid of them. Deputy Head Evans (Faith Brook) warns that while they are mostly good kids, they come from rough homes, and excel at riding roughshod over teachers. Headmaster Florian (Edward Burnham) explains that they are principally rejects from other schools and their unruliness is what drove their last teacher to resign.
The students more than live up to their reputation. Led by Bert Denham (Christian Roberts), and Pamela Dare (Judy Geeson), the pupils are an unruly mob who views the classroom as their domain, not Thackeray's. A battle of wills ensues - Thackeray tries to establish order, and the students resist his authority. As the students' antics progress from simple disruptive behavior to embarrassing Thackeray by rigging his desk to collapse, his calm manner and resistance to baiting earn him some early respect with the class.
One fateful day, a water balloon dropped from the window of his classroom narrowly misses Thackeray as he enters the building. In the classroom, he discovers something; presumably a sanitary pad, burning in the grate. He finally loses his composure, ordering the boys out of the classroom and berating the girls for their disgusting behavior. Fearing he's made a fool of himself, he retreats to the staff room, expressing his dismay that he has been so easily manipulated by 'these kids,' then realizes that treating them as kids is precisely the mistake he has been making with his students.
Thackeray returns to the classroom to outline a new approach and set some ground rules. The students will be leaving school soon, and will enter a society where neither sluts nor hooligans prosper. Therefore, he will treat them as adults, and allow them to decide what topics they wish to study. He emphasizes this by throwing out all the textbooks, and insisting that they will be expected to use proper forms of address (both toward him and amongst themselves) and to take pride in their appearance and deportment.
Denham continues to bait Thackeray, but the rest of the class is won over. Although Thackeray's humble background is quite like their own, he's made a success of himself by cultivating his language and dress. The students are especially excited when he suggests that they should go on a class outing to the museum. The Headmaster approves the outing, and Thackeray arrives on the morning of the trip to find a classroom of well-dressed, well-scrubbed students. The outing is depicted as a photomontage of happy, wondrous students having the time of their lives. During the bus ride back to school, some of the girls idly speculate whether a personal relationship is budding between Thackeray and Gillian (who volunteered to accompany the group on the outing). But as their classroom environment approaches perfection, the outside world infringes, threatening to derail everything.
Their gym teacher, Mr. Bell (Dervis Ward), insists that 'Fats' Buckley (Roger Shepherd) participate in vaulting, despite his classmates' objections. The vault collapses under Buckley's weight, and Potter (Christopher Chittell) threatens Bell with the vault's broken leg. Thackeray is called to defuse the situation. In class, he demands that Potter should apologies for the incident, pointing out that if a knife or gun was available, things could have been much worse. Potter only agrees when Denham points out that Bell will press criminal charges if he doesn't apologies, and that Potter will need Thackeray's recommendation for a job interview when school ends.
Thackeray has obviously lost the support of much of the class, especially the boys. They refuse to invite him to the class dance. When Seales' (Anthony Villaroel, the only black student in the class) mother dies, the class takes up a collection for a wreath, but refuses to accept Thackeray's donation. The girls still seem to respect him, though. It is clear they feel uncomfortable delivering the wreath personally, due to racial issues, but Pamela offers to take the flowers to the funeral, over Denham's objection.
Thackeray finally receives a job offer, while the Headmaster advises him that he feels 'the adult approach' has failed, that future class outings are cancelled, and that Thackeray will take over the boys' gym classes.
Pamela's mother (Ann Bell) comes to speak with Thackeray; concerned that Pamela is staying out late and might be getting into trouble. When Thackeray speaks with Pamela, she insists her mother does not care about her, and that the presence of male callers at her mother's house excuses her actions. Thackeray maintains that Pamela still owes her mother respect, and disillusioned, she angrily accuses him of being 'just one of them'. She refuses to take the flowers to the funeral, either. Thackeray's split with the class is complete - now they all hate him. Due to this, Thackeray considers resigning, but decides to keep going.
In gym, Denham insists they have a boxing class, beginning with him and Thackeray. Thackeray reluctantly agrees. Despite getting in some early blows, Denham is disabled when Thackeray knocks the wind out of him. Thackeray draws back his fist to hit Denham again but gains control of his emotions and declares the fight over. Afterward, Denham waits to speak with Thackeray in the stairwell. Although Denham admits he was trying to hurt Thackeray in order to convince him to resign from teaching, Denham is surprised that Thackeray did not capitalize on his advantage. For his part, Thackeray admits that he lost his temper, but that he understands the apparent unfairness of some of his decisions. Thackary then offers Denham a position as a boxing instructor to the younger students next term. Surprised that he would be regarded as a potential teacher, Denham nevertheless promises to consider it. Denham is clearly impressed, and expresses his admiration for Thackeray to his fellow students. By winning Denham over, Thackeray wins back the respect of the rest of the class, too. He is invited to the class dance, and when he shows up at the Seales funeral, is greeted by the entire class who have come to pay their respects.
At the dance, all has clearly worked out well. Weston admits that Thackeray is really quite a gifted teacher, and should reconsider leaving. The Deputy Head concurs, suggesting that he should go to another school, if nothing else.
Barbara Pegg (Lulu) announces a 'ladies' choice' dance, and Pamela singles out Thackeray as her partner. Denham announces that the class has 'something special' for Thackeray, and Miss Wong (Lynne Sue Moon) presents him with a pewter mug. Thackeray is too moved for words, and retires to his classroom.
Two young students storm into the classroom, mocking his gift and joking that they'll be in his class next year. When they leave, Thackeray rises, ponders his situation, and then retrieves the job offer from his pocket. He tears the offer up and throws it in the wastebasket.