[evanny]

you have a point that a student won't know if he is thought incorrectly by a non-native.
however i can't imagine any big mistakes that could be made and then hard to re-learn. mistakes usually made by non-natives are either pronunciation or simple translation - forgetting the meaning of words. other than that i don't know if any major mistakes that would "damage" students language in a long run can be made by a non-native, especially since all non-native speakers have finished universities in order to get a license to teach.