Shar’i description of hijab and niqaab, and should the principal tell the teachers and students to wear it?
Question

                I am a manager of Quran memorizing school. I have ordered the teachers not to wear the see through niqab or the two layer see through one. But they refused and said that it is not haram. How shall I stop the students wearing it; as they intentionally raise one of the two layers up after they leave school, and also in transportation. When I ask the students to wear the full covering niqab they refuse because the teachers are not doing this themselves. I wish I receive a detailed answer from you.
Praise be to Allah.
    Firstly: 

    May Allaah reward you with good for your eagerness to cover 
    and remain chaste, and to make precautions, and to advise and guide the 
    teachers and students. This comes under the heading of fulfilling the 
    trust. 

    Secondly: 

    We cannot say that it is obligatory for a Muslim woman to 
    cover the face with a multi-layered khimar. What is required is for her to 
    cover the face, whether it is with a single layer or several layers. 

    Islam allows the khimaar and the niqaab. Those scholars who 
    disallow the niqaab do not do so because it is not prescribed in principle, 
    rather it is because some women choose a type of niqaab that does not meet 
    the  shar’i conditions, such as making the opening for the eyes too big. The 
    same may also be said of the khimaar. It is not disallowed in and of itself, 
    but because of its nature, if the fabric is too thin and the face can be 
    seen through it.  

    The scholars of the Standing Committee were asked about the 
    Islamic ruling on the niqaab. They replied:  

    As for the niqaab, Abu ‘Ubayd said, concerning the definition 
    of niqaab among the Arabs: It is that from which the socket of the eye 
    appears, and they used to call it al-waswasah or al-burqa’. As for the 
    ruling concerning it: it is permissible. It was originally mentioned in the 
    report narrated by ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) 
    who said: “A woman in ihraam should not wear niqaab or gloves.” According to 
    another report he said: I heard the Prophet  (peace and blessings of 
    Allaah be upon him) forbid women to wear gloves when in ihraam. 

    The fact that the Prophet  (peace and blessings of Allaah 
    be upon him) forbade the woman in ihraam to wear niqaab indicates that it is 
    permissible to wear it when not in ihraam. Moreover it cannot be understood 
    from this hadeeth that it is permissible for the woman in ihraam to uncover 
    her face if non-mahram men can see her; rather she must lower the khimaar or 
    niqaab (over her face) until they have passed her. The basic principle 
    concerning that is the report narrated by Imam Ahmad, Abu Dawood and Ibn 
    Majaah from ‘Aa’ishah (may Allaah be pleased with her) who said: The riders 
    would pass by when we were with the Messenger of Allaah  (peace and 
    blessings of Allaah be upon him) in ihraam. When they drew near to us one of 
    us would lower her jilbab from her head over her face, and when they had 
    passed by we would uncover (our faces). End quote. 

    Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh ‘Abd al-Razzaaq 
    ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn Qa’ood. 

    Fataawa al-Lajnah al-Daa’imah 
    (17/171, 172). 

    Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have 
    mercy on him) was asked: 

    What is the ruling on the burqa’ (niqaab) if it is not worn 
    for adornment, but for covering, and also wearing a cover (on top of it). 

    He replied: 

    There is nothing wrong with it, because it will not be seen, 
    as it is going to be covered with something over it. But as for the burqa’ 
    that is on the face and is not covered, we do not say that it is 
    permissible, because it is fitnah and because women do not limit themselves 
    to that. If women would stick to the eye opening then we would say that this 
    is niqaab, which was known at the time of the Messenger  (peace and 
    blessings of Allaah be upon him) and there is nothing wrong with it. But 
    believe me, if you say that it is permissible for women to wear niqab up to 
    the eyes and look out from behind the niqab, within a short time the hole in 
    this niqab would widen to include the forehead and cheeks, then the cover 
    would decrease until the whole face is exposed. This is well known to be the 
    way of women, so shutting the door to that is the best way. End quote. 

    Liqa’aat al-Baab il-Maftooh 
    (14/question 43) 

    Based on that: 

    We say to the teachers, sisters and all Muslim women: whoever 
    among you wants to cover her face has chosen that which is most concealing 
    for herself, and that which is better for religious commitment. But she has 
    to adhere to the prescribed shar’i form of khimaar or niqaab. It is not 
    permissible to make the opening of the niqab to show more than the eyes, and 
    it is not permissible to make the khimaar so thin that it shows the face. 

    It was narrated from ‘Alqamah ibn Abi ‘Alqamah that his 
    mother said: Hafsah bint ‘Abd al-Rahmaan entered upon ‘Aa’ishah the Mother 
    of the Believers, and Hafsah was wearing a thin khimaar. ‘Aa’ishah tore it 
    and gave her a thick khimaar to wear. 

    Narrated by Maalik in al-Muwatta’ (1693) and 
    al-Bayhaqi in al-Sunan (2/235). Its isnaad is hasan.  

    Ibn ‘Abd al-Barr said: 

    Any garment that shows the shape and does not conceal, it is 
    not permissible to wear it under any circumstances, except with another 
    garment that does conceal and does not show the shape, for the one who wears 
    it is in effect naked, as Abu Hurayrah said, and this is narrated from the 
    Prophet  (peace and blessings of Allaah be upon him) in the hadeeth of 
    Abu Hurayrah. End quote. 

    Al-Istidhkaar (8/307). 

    One other thing: Your responsibility in the Qur’aan 
    memorization centre is to make the teachers and students observe the 
    covering that is required of them inside the centre. As for what happens 
    after they leave the centre, or before they get there, is not your 
    responsibility, but you should advise them and teach them, without forcing 
    them to wear something in particular. 

    We ask Allaah to guide you to all that is good, and to guide 
    the teachers and students to that which is good and proper. 

    And Allaah knows best.