All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Covering the face with Niqaab is something obligatory according to the preponderant opinion of the legal scholars. We have previously clarified this in Fataawa 83033 and 83702. The Muslim woman must adhere to it whether she is in an Islamic state or in a non-Muslim country. Her mere presence in such a country does not permit her to remove the Niqaab. The same is said regarding studying or failure to wear it due to some weakness or frailty. All such instances do not allow for its removal.
However, if you fear some definite harm or if you need to study and cannot find another place for lessons free of such a prohibition, then there is no sin upon you for removing it. In such a case, you should stay at home and not go out except for necessity.
Promising Allaah that you will do something or leave something is considered an oath according to the preponderant opinion of the Muslim jurists. This oath of yours is an obedience to Allaah; so as long as you did what you promised, i.e. you at least started to wear the Niqaab, then you have fulfilled your oath without further obligation on your part.
Allaah Knows best.