the best questions will probably be those that are tailored to that particular school. maybe ask about the boards? ask about research at that school, or the interviewer's research? i've also asked, "What new advances do you anticipate in dentistry over the next 5 years?" "What about my interview/application can i improve on, in your opinion?"
i like that last one because it gives them a chance to give you feedback so you can better tell how you did, as well as helping you do better in future interviews. good luck.