1) My father is a school Principal
- if you were already speaking about the school, then people would infer he was the principal at that school, and you could omit the school before principal.
2) My mother is a teacher.
- Again most people will infer it is a school teacher in America. They may follow up with a question asking. Which school?
3) similar to teacher, just say: My sister is a student. Follow up question will allow you to be more specific for a elementary, high school, college/university and the name of that school.