Are the teachers at your placement school nice? Mine were always very positive when they heard I had an interview, and were willing to talk through my ideas for the interview lesson and give some advice. If someone at your school could do the same for you, it might help you feel a bit more confident. Also, don't go in there with the hope of getting the job - see it more as a practice for the 'real thing'. That way, if it doesn't go so well it doesn't matter, because you just know what you need to improve for your next interview, rather than worrying about how you messed up your chances at getting that job. I learned a lot from my first interview about what to expect, and was much more confident for my second one as a result. Good luck!