What do the boys want Braithwaite to do in To Sir, with Love?

The boys want Mr. Braithwaite to box Denham and answer the boy's challenge. Described as a "husky blasé" student, Denham is insolent and defiant. He is "watchfully hostile" to his teacher, and makes snide remarks whenever he can. One day Braithwaite notices that Denham and his friend Sapiano are snickering at something which Denham hides inside his half-open desk. When Braithwaite pulls open the desk, he discovers that Denham has a pornographic magazine with an enlarged picture of a scantily clothed woman. When his teacher takes this photograph from him, Denham smiles insolently at him, having intended that Braithwaite discover it. After tearing up the picture and starting back to the front of the room, Braithwaite hears Denham's racial insult, and he realizes that something will happen soon.
On Thursday of the week in which Denham's picture was taken from him, there is an air of expectancy and excitement in the classroom, and at recess students stand in small clusters and whisper. Then, in the afternoon physical education class, Braithwaite notices that Sapiano has his arm bandaged. When he asks the class to line up, Denham, who is interested in boxing and is in great shape, requests that they have boxing first this day. Agreeing to do this, Mr. Braithwaite instructs the boys to arrange themselves in pairs according to their sizes. Soon, Denham points out that his sparring partner Sapiano is hurt this day, and he asks if he can box with his teacher. 

"Go on, sir, take him on," the students encourage their teacher."No, Denham, I think you'll have to skip it for today." (Ch. 11)

Then, Denham takes off his gloves and he drops them by Braithwaite, who can read the disappointment in his students' faces, as well as their disgust because they believe he is afraid. Braithwaite changes his mind, saying, "Okay. Let's go." When they box, Braithwaite tries mainly to dodge the jabs of Denham, who is a good boxer. "Come on, Sir, go after him," Patrick Fernham calls out. Suddenly, Denham punches his teacher hard in the face; the blood he has drawn and the encouraging shouts to "Go after him" from students incite Braithwaite and he punches Denham solidly in the solar plexus. When this blow doubles the boy over and he collapses on the floor, the other students react as though their teacher has "suddenly grown up before their eyes."
Ordering everyone else to line up for vaulting, Braithwaite takes Denham to sit against a wall. After class he tells the boy that he simply landed "a lucky punch," and he suggests that Denham soak his head in some cool water.

This incident marked a turning point in my relationship with the class. Gradually, Denham's attitude changed, and...that of his cronies....now [Denham's comments] were more acceptable to all of us, for they were no longer made in a spirit of rebellion and viciousness. (Ch. 11)

This incident is, indeed, a turning point as Braithwaite receives the students' obedience and respect from then on. Denham still makes some derogatory comments, but the venom and bitter sarcasm is no longer in his words.