I'm an Apple Campus Rep for Temple University (I assume that's what you mean by Student Rep). It really is a great job, it feels great to be the "mac guy" on campus. I can't tell you exactly what you will do because it will be up to your boss, but I can tell you what I've been doing and what else could be done. I talk to students, hand out promo cards (until the end of March, we have these iTunes scratcher cards to give out...everyone is a "winner"), I'm starting a Mac User Group on campus, and eventually I will have to set up demo tables (set up includes flyers, CDs, and a mac or two on display).
As for the interview, don't sweat it. As with most things, be yourself. Answer questions sincerly, and dont act like all you do is talk about Macs. I actually said in my interview that I recognize the strengths and weakness of Windows and OS X. They are looking for someone who knows a thing or two, willing to learn (they have a great online training site for all apple products), willing to go out there and get the word out. All in a 15 hour week, hehe. With my other on campus job, my classes, I still find/make time for Apple.
If you have any other questions, I'll be more than happy to help you. E-mail me at
dbry@temple.edu or
david.bry@mac.com
I hope you get the job, good luck!