I think most math teachers are rational. You can ask some questions about his or her current class. Some teachers will be more excited than usual, but you'd better say something happy, but try not to say things that are easy to get angry. How many math teachers can guess! You can also ask him some questions about life. At least our math teacher there welcomed it very much. If you really want to get the news, it is better for him (her) to say it willingly. If it really doesn't work, I suggest you find someone in the class he or she is trying to help now!