What is the ruling on a teacher retracting his oath not to teach a specific student, after the student and his family pleaded with him and he promised not to cause trouble?

As long as the student has regretted their action and their family has come requesting their return, it is better to break the oath and offer expiation for it, based on the Prophet's (peace be upon him) saying: "If any of you takes an oath and then sees something better than it, let him offer expiation for it and do that which is better." The expiation for an oath is given as a choice between: feeding ten poor persons, or clothing them, or emancipating a slave. If one cannot find any of these, then fasting for three days, based on the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or freeing a slave. But whoever finds none - then a fast of three days. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful."