The incident that is the last straw for Mr. Braithwaite and that marks a turning point in his relationship with the unruly students that he has been tasked with teaching is when some of his students burn a used sanitary napkin in the classroom. This incident, which takes place in chapter 8, forces matters to come "to a head" and leaves Mr. Braithwaite horrified.
The incident in question takes place during recess one day, when Braithwaite returns from going to get a cup of tea in the staffroom and discovers "the classroom smoky" as the result of an at-first-unidentified object burning. Numerous students are standing around, completely unbothered by the fact that an article, which Mr. Braithwaite soon realizes to be a used sanitary product, has been thrown into the fire and "an abortive attempt" has been made to burn it.
He reprimands the girls for their unladylike behavior, turning "the full lash of [his] angry tongue" on them and demands that they remove it. However, he soon realizes that his display of temper will do nothing to change the difficult dynamics in his classroom.
After this incident, Braithwaite realizes that he is going to have to implement some radical changes in order to get through to these students. It is at the beginning of chapter 9 that he has an idea and begins to introduce respect by insisting that he be called "Sir" and that every girl in the class be referred to as "Miss."