Be there for them but do not make choices in their place.
Teachers: students do not come to yoga to be preached upon. Giving them a lecture on day one about how worthless they are if they are not vegan will most probably make them flee and rightfully so. Always remember your own path, how long it took you to be who you are today and let your students reinvent themselves and their relationship with practice and food at their own pace. Be there for them but do not make choices in their place.