What’s the Halacha re a woman taking self-defense classes taught by a man? The class would be semi-private – a male instructor with 2 female students. The instructor is a jewish Israeli guy who is traditional on terms of observance. (I don’t know much about the classes but I assume there must be physical interaction).
There are a few concerns with such a class. Firstly, one man with 2 women is a problem of Yichud, assuming this will be taking place in a private place. Physical contact between the 2 genders is prohibited, especially here where it would be close up bodily contact to teach positioning etc.. In addition the tznius is likely an issue as the females will need to be in exercise clothes, which usually includes pants and other tight fitting clothes.
Classes such as this should be taught by a same gender instructor.