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 is His slave and Messenger.
In principle, it is permissible for a man to teach women religious studies, there is much evidence on female companions reporting from male companions and vice-versa. However, the religious obligations should be met when doing so; the woman should wear her proper Hijab, she should not apply perfume, the teaching should be administered behind a curtain, and this teaching should not lead to a forbidden matter, like being in seclusion with a woman or the women speaking in a soft and tender voice and the like.
The fact that the female students are unveiling their faces, then this leads to temptation because the woman's beauty is in her face. So they are obliged to cover their faces according to the most preponderant opinion of the scholars especially if they are very beautiful. So, it is not permissible for a man to teach women if they do not cover their faces unless there is a curtain between them and the teacher. This ruling applies both to an individual woman or a group of women.
Allaah knows best.