There is no script. It’s just a conversation. A “successful” interview is one in which you demonstrate ease in conversing with an adult and genuine interest in what you’re talking about, whatever that is. The interviewer will throw some softballs to put you at ease and get you talking. What those questions might be is irrelevant. They’re trying to get a sense of “you” and how ready you are for the boarding school experience and how well you would fit into their community. Nothing more. Just relax and enjoy the conversation.
If you don’t know the answer to this question, you have more research/introspection to do as it is the reason you are applying. YOU need to be able to articulate this answer. No one else can supply it for you as what makes any school special to any applicant is as varied as the applicant. This answer is yours and yours alone.