It is important to consider the amount of background knowledge that a student of a given age has. "Night" could be read by fairly young students, but students without knowledge of the Holocaust would miss out.
If the History department at a given school teaches an Apartheid unit, Kaffir Boy should definitely be taught in the same year. I find that kids without a pretty good knowledge of Apartheid get frustrated with textual elements that they don't understand.
Of course, the English teacher can always give historical background (I do, and love doing it)! The teacher should consider the ability of his/her students to understand the complex ways in which Apartheid legislation actually affected individuals.
A thought should be given to graphic content, especially the "prostitution scene". I decide whether or not to teach the book to a given class by deciding if the students can handle this question (in terms of maturity and analysis):
Mathabane notes that child prostitution was a known practice in the townships. If parents like Johannes' know about it, why aren't they keeping a closer eye on their children? Why aren't they getting together with other parents to prevent it?