Is the teacher sinful if he teaches law to students, some of whom may work in fields that violate Islamic law?

It is permissible to teach and study man-made laws with a correct intention, such as for the student to benefit from them in ways that do not contradict the Sharia, and to recognize the superiority of Sharia rulings over them, while disliking and abhorring governance by them. This is supported by Hudhayfah's inquiry about evil; so that he may avoid it.