The most obvious difference between a prep school and (many) university interviews is that rather than a presentation of your research they will drop you into a classroom full of students and turn you loose. What they want/expect you to do will vary widely depending on the school, as will the amount of guidance you receive. Sometimes you'll receive a specific topic, other times you'll get little or nothing to go on. Some tips to make things go more smoothly:
- Bring a baggie full of large name plates (the ones that fold into tents) and sharpies. As students come in, ask them to write their names. This will make it much easeir to call on students without resorting to, "You, in the blue sweatshirt."
- If you are going to lead a discussion of some sort, make it a self-contained unit. Bring a copies of a short reading, and have them do it in class. (Better yet, have them read aloud.) You could try leading a discussion of the reading assigned by the regular teacher, but you're betting your job that the students did the reading, and if they didn't you'll be out there flapping.
- Treat the class like the first of the semester. (No, don't go over the syllabus.) They don't know you, and don't know your shtick, so explain what you are going to do in class. Also, if you are prone to excessive enthusiasm, tread lightly. (I'm kind of loud and might have scared the 9th graders.) Be 80% of yourself.
- Write on the board. (You might bring your own dry-erase marker for this. You don't want your class torpedoed by an equipment malfunction. Incidentally, what do you call it when a dry-erase marker runs out? They can't dry out, can they?)
A second issue to keep in mind is that department politics in a prep school can be quite different than a college or university. In large part, this is a matter of scale, and the significance of a single hire to a department. From a political perspective, at all but the smallest colleges, your arrival in will probably not be particularly significant. By contrast, at all but the largest prep schools you will be one of four or five people in the department, so your arrival will be tremendously significant.
For example, many prep schools are rethinking their AP offerings in the humanities, and some APs are on the chopping block. While you might not have much sympathy for standardized testing, and hate the idea of teaching to the test, these classes have been around for a long time, and inevitably have strong support among some members of the department. In a small department, your position on this issue will likely determine the future curriculum. As a result, the way you answer a question such as, "What do you think of the AP?" will shape the way different members of the department view your candidacy. My argument here is not that you should avoid answering this sort of question ("Gee, I haven't thought about curricular issues" will get you nowhere), but you should know why people care intensely about your answer.
Beyond this, the prep school interview will feel quite familiar. You'll get a nice dinner, meet a bazillion people, and get a bazillion different versions of the same questions (all focused on teaching).
And don't worry - you'll do great.