In what chapter of To Sir, With Love does a student come into class late because he/she had to take care of something at home? (Not Pamela Dare in the Chapter 9)

There are three instances in To Sir, With Love, in which someone comes to class late, having either been held up somehow or having had to handle something before coming. One of these happens to Ricardo Braithwaite himself, the teacher otherwise designated as "Sir" for most of the novel, when he is delayed by a late train (chapter 12). The second instance is the one referenced in the question, when Pamela Dare is late (for unknown reasons) and dashes breathlessly into the classroom, unwittingly becoming an example for Braithwaite's lesson that day (chapter 9).
The third instance of tardiness in the novel happens in chapter 20, when the student Larry Seales arrives late to class one day in December after his mother's unexpected death, having been helping his father at home make arrangements for the funeral and needing several more days off to continue to do so.