Praise be to Allah.
May Allaah reward you for your efforts in giving useful lessons to these Muslim students in your city, who are in the greatest need of learning about their religion in a land of kufr. Perhaps your background of having lived in Saudi Arabia will help you with this. I advise you to concentrate on teaching them about Tawheed and correct Islamic ‘aqeedah (belief), explaining how to do acts of worship from a Fiqh point of view, and teaching them the biographies of the Prophet (peace and blessings of Allaah be upon him) and his Companions.
As for the matter which you raised in your question, letting boys and girls sit together facing one another in one halaqah is not free from dangers, as you know. I think that you feel the same way, which is what made you ask this question, especially when we expect that the hijaab of the girls where you live is incomplete and that the faces of some of them, at least, are uncovered in front of the boys. For this reason I suggest one of two things: either get the girls to sit behind the boys, wearing full hijaab, so that you don’t feel embarrassed when you face the group to give a lesson; or put up a barrier or screen between the boys and girls, such that they can still hear your voice, if the girls do not want to wear full hijaab. Then they can ask questions from behind the screen, so long as they are not soft in speech, or they can write questions on pieces of paper.
May Allaah help us and you to do good and to call people to His way and earn His pleasure. May Allaah bless our Prophet Muhammad.