A model interview for a High School Principal or Head Teacher would partly follow the same lines as that for any trusted professional joining a new establishment, but there may be extra considerations to reflect the fact that the job would be for an educational establishment. For example, think about the sorts of questions usually asked of professional roles. Asking the candidate about their vocational motivation would be good, as well as questions covering their past employment history. These have become less formal of late, with interviewes being asked 'tell me about a time when....' rather than 'give us an example of your professional judgement being proved right ' and so on.
You might want to include some questions designed to draw the candidate out, questions where he or she opens up about their personality. These questions are often in te guise of enquiries about hobbies, sports, clubs, volunteer work and professional affiliations. The idea here is to elicit responses which illuminate the candidates dealings with others (peers,employees,students,state bodies, parents and so on.) Employers like open, out-going friendly staff who are approachable and easy to read. More isolated, quiet and reclusive types might not be suited to a school which is part of a local community hub for example.
Think about the students and parents and what they would want to know about safety, security and responsibility. The children must be safe in school and this requires diligence and thorough working practices and leadership. In some schools, an older student representative is allowed to ask a question!