1- the issur is only to TEACH a gentile.
2- as far as lifnei iveir there are choshuver shittos (probably the shach and tosfeis chagiga) will probably hold it is only a problem if you put the gemorra into a gentiles hand when he told you he is in a mood to learn now.
3- rav moshe is matir in the teshuvos to teach a class in a day school even if there is some non-jewish students in the class. there.