While saying any of the sentences with at school in the end is not incorrect, I think we more commonly say a school principal, a school teacher or a student.

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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.