What part of chapter 9 shows the theme of social codes in To Sir, With Love?

To Sir, With Love, by E. R. Braithwaite, was written in 1959 and demonstrates the social codes of that time. E. R. Braithwaite based his novel on his own experiences teaching as an African American man in a predominately white school. In chapter nine, Mr. Braithwaite, or "Sir," explains the social codes his class will follow in order to learn how to live in an adult world, outside of his classroom. Sir states,

I have decided that from now on you will be treated, not as children, but as young men and women, by me and by each other. When we move out of the state of childhood certain higher standards of conduct are expected.

This changes the power dynamic of the classroom and allows his students to decide how they want to be treated. Sir then gives his students concrete instructions on how to act and be treated as an adult. Sir explains, "as from today there are certain courtesies which will be observed at all times in this classroom. Myself you will address as 'Mr. Braithwaite' or 'Sir'—the choice is yours," which again gives the students a sense of power over their lives. Choosing how to address someone in power, when both choices are appropriate, rather than being told, allows his students to recognize their own autonomy within social codes of conduct. However, Sir does not give the students a choice when he explains how they shall address each other: "The young ladies will be addressed as 'Miss' and the young men will be addressed by their surnames." This creates a sense of equality and respect among the students.
Equality, respect, and power seem to be the main themes of chapter nine. Students learn that words and appearances matter when you address others. They learn that in order to be treated as adults, they must first act and treat others as adults. This not only includes how they speak in public to those in authority, but also how they speak to their peers. The students also learn that how they look is important and that

there is nothing weak or unmanly about clean hands and faces and shoes that are brushed. A man who is strong and tough never needs to show it in his dress or the way he cuts his hair. Toughness is a quality of the mind, like bravery or honesty or ambition; it has nothing whatever to do with muscles.

Sir is attempting to explain that social codes are observed not only in words, but in how the students present themselves.