Teeth are referenced in many of the chapter titles of the novel ("Teething Trouble"; "Molars"; "Canines: The Ripping Teeth"; etc.) and also appear in as references and as cultural markers within the text. As a novel about integration, immigration, ethnic culture and identity and various strains of resistance to each of these things, cultural markers bear a special significance.
At one point in the novel, Millat, Magid and Irie are lectured by an old man about his experience in war in Africa. He recounts a story about how it was so dark in the jungle that the soldiers looked for enemies by trying to spot their teeth. The Africans fighting in this war had such white teeth they were easily spotted and killed.
This episode suggests a number of ideas relating to the novel's themes. 1) The Caucasian man telling the story is insensitive to the fact that he is telling the story to ethnic minorities who will identify with the soldiers with the "white teeth" more than they will with him, an old white man who was afraid to let them in to deliver their charity stuff. 2) The children band together, despite their differences, because they share a common situation as immigrants, as minorities, and as neighbors. 3) History is personal, often divisively so.
Later in the novel, the high school that Millat and Irie attend is described as being populated by people who smoke. A long section of prose is devoted to this notion and ends with a comment on how the smoke is turning "white teeth yellow". As the details of smoking are decidedly cultural on a local level, they may also be seen as an indicator of a larger self-defining cultural trend among the ethnic mixture of people at the school.
Smoking, as an act, is a means by which the students assimilate into a culture of their own design, symbolically eliminating the marker of their isolation and difference (white teeth) in the process.
Teeth then can be seen as relating to these themes of assimilation and ethnic identity.