Praise be to Allah.
It is not permissible for a woman – especially if she says that she wears niqaab – to mix with non-mahram men and to sit with them, whether that is in school or at work. We have stated the ruling on mixing and its evil consequences in our answers to questions no. 1200, 20784 and 12837.
The evil consequences of mixing include men and women looking at one another, which is haraam. Allaah has commanded the believing men and women to lower their gaze and avoid looking at that which is not permissible for them.
It is not permissible for non-mahrams to see anything of her, or for her to be careless about how she dresses so that anything appears of her that it is not permissible for her to show.
Shaykh al-Islam Ibn Taymiyah said:
The truth of the matter is that Allaah has made two types of adornment: visible and invisible. It is permissible for a woman to show her visible, outward adornment to people other than her husband and mahrams. Before the verse of hijab was revealed the women used to go out without a jilbaab and the men could see their faces and hands. When it was permissible for a woman to show her face and hands, it was permissible to look at them because it was permissible for a woman to show them. Then when Allaah revealed the verse of hijab and said (interpretation of the meaning):
“O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies (i.e. screen themselves completely except the eyes or one eye to see the way). That will be better, that they should be known (as free respectable women) so as not to be annoyed. And Allaah is Ever Oft-Forgiving, Most Merciful”
then the women began to observe hijab in front of men.
The jilaab is a wrapper, which Ibn Mas’ood and others called the rida’ (cloak), and which the common folk [i.e. at the time of Ibn Taymiyah] call the izaar. It is a large izaar with which a woman covers her head and the rest of her body. Moreover it may be said that when they were commanded to wear the jilbaab lest they be recognized, this refers to covering the face, or covering the face with a niqaab (face veil), hence it may be understood that the face and hands are part of the beautify which women are commanded not to show to non-mahrams. So there is nothing left that it is permissible for non-mahrams to see except the outer garments.
So the face, hands and feet are not to be shown to non-mahrams according to the more sound of the two scholarly opinions, unlike what used to happen before the abrogation, indeed nothing is to be seen but the garment.
Majmoo’ al-Fataawa, 22/110-114.
And Allaah knows best.