Alright, I'm going to give you some serious advice:
Preparation:
* Know yourself - your strengths, weaknesses and accomplishments. They might ask about these, so thinking about them beforehand will help.
* Know them. Know what the school is known for, know where they're strong. Know what kind of clubs, competitions, scholarships, social activities they do. Nobody wants to interview someone who isn't even sure where they're interviewing for.
* Dress: Yes, you should dress smartly, but it's also very important for you to be comfortable in what you're wearing. If you're comfortable in a suit (and you have one), then great! Wear that. If not, wear the smartest thing you have that you can be comfortable in. Wear something that you think will fit the model of the person they're looking for. (LPT: Get comfortable in a suit. No-one ever interviewed poorly because they were overdressed).
* Arrive 15-30 minutes early. If you're late (or even cutting it fine), you will be unnecessarily stressed. If you're early, they won't be waiting on you, and you have time to relax.
* Smile. Be friendly. Look people in the eye. Listen when they're talking.
* Talk yourself up when asked about your skills, but don't be arrogant. As in #1, show that you know where your strengths and your weaknesses are.
* Remember that interviews work both ways. You're interviewing them to find out if the school is right for you, too. Have some questions for them.