When Philip comes to sit his winter term English exam, he's required to answer a question on The Call of the Wild by Jack London. It's already been established that Philip doesn't think much of the book, finding it insufferably tedious and boring. So when it comes to answering a question on his least favorite book, we can guess that Philip will take yet another opportunity to goof off, and that's precisely what he does. In writing that Buck is actually symbolic of a cat, Philip is showing once more his inability to take his studies seriously. Miss Narwin responds by giving him a C minus. She tells Philip that, although he's lazy, he does have the potential to do better work. That explains why she gives him a much higher mark than his blatant disrespect and insolence would ordinarily warrant. Miss Narwin's trying to encourage Philip to do better, and the grade she awards him is a reflection of this.