Is it permissible to retract an oath to deduct half a degree from disruptive students and to atone for the oath?

If you find it in your best interest to break your oath, then you may do so, and you must offer expiation (kaffarah) for it. This is based on the Prophet, peace and blessings be upon him, who said: "Whoever takes an oath and then sees something better than it, let him do that which is better and offer expiation for his oath." Your oath is binding, and you cannot be released from it except by offering expiation.