Frank does not encourage Taplow’s comments on Mr. Crocker-Harris. It is possible that he just wanted to have an insight of what students think of his colleague. We cannot neglect the fact that Frank was a teacher himself. However, he was much understanding towards his students and thus, students opened up to him quiet easily. Frank is young and shows a better understanding of the behaviour and psychology of a student and so is humble towards them.
However, it is quite clear that Frank is a teacher who does not appreciate sarcasms on a fellow teacher. Therefore, he points out to Taplow that he got too far in mocking his teacher. Besides, when Taplow casually calls Mr. Crocker-Harris as Crock, Frank corrects him.