There are two questions here. First, will Kaffir Boy cause controversy if it is taught to younger teenagers? Second, are younger teenagers capable of handling the themes of the book?
The teaching of this book is bound to cause controversy, perhaps not all the time, but at least some of the time. Many parents want to protect their children from the worst of the world's cruelty and horror, and Kaffir Boy describes such cruelty and horror in detail.
Personally, I believe it is important for kids to learn about difficult issues, even if they are upsetting. I think it is appropriate to start teaching kids about these issues from the time they are twelve or thirteen--old enough to understand and evaluate complex topics. Most kids would be helped, not hurt, by thinking carefully about a book like Kaffir Boy with guidance from a supportive adult.
However, there are exceptions to every rule, and some kids are more sensitive than others. If some students are truly overwhelmed by Kaffir Boy, teachers could consider offering them an alternate assignment.