Summarize the movie To Sir, With Love.

To Sir With Love is an award-winning 1967 British film set in a secondary school in contemporary London's East End. Sidney Poitier plays Mark Thackeray, a man of color from British Guiana who is pursuing an engineering career but agrees to take a teaching job in a school populated by teens with behavioral and academic issues. The film's themes have mostly to do with social class and race.
The teenagers continually push the boundaries of acceptable behavior, and Thackeray is patient with them until things escalate and the behavior crosses a line that he will not tolerate. After that event, Thackeray changes his way of interacting with the students, treating them as adults and helping them with not only academics but social and life skills. He continues to struggle with the students but does not give up on them even when they mistreat him.
Eventually, Thackeray is transferred by the school's head to a position as a physical education teacher because the school head doesn't believe that Thackeray is making progress with his students in the academic classroom. After a boxing match with a particularly recalcitrant student, Denham, Thackeray's former students show him respect and affection. Though he has, in the meantime, received an offer for an engineering position, Thackeray decides to return to the school for the next year.