I can only echo what others have said.
I would also recommend something that may sound daft to a lot of people, but it does help (imo): really think about what you're going to wear to an interview if you're in a school that allows you to wear your own clothes. I'm usually quite a smart dresser - I preferred to wear pretty skirts and court shoes to wearing jeans all the time, and tbh I think that dressing as I usually did on the day of my head girl interview was a huge disadvantage. Obviously it's not the only thing they take into account but how you dress is a huge reflection on your personality, and if maybe I'd thought to dress as a 'normal' sixth former (eg. jeans and a jumper or something like that) they might have viewed me as being more normal and more able to fit in with my peer group. There were five of us that went for two head-girl positions. Two were nice enough but didn't really do anything for the school or get particularly good grades. Then there was a girl who I shall call girl A, who had been in loads of concerts and school plays, was really nice and quite clever. Then there was girl B, who was confident and popular but a real bitch with it and not that clever (although in her defence she was quite involved in sports at the school). Then there was me who, like girl A, had been in loads of concerts and school plays, helped with PTA events, had wanted to be head girl for as long as I could remember, got good grades and didn't get into trouble. Many people said that they expected the two posts to go to me and girl A, and they didn't - they went to girl A and girl B. When girl B came out of the office and said excitedly that she was head girl I honestly thought she was joking, and it upset me hugely not to have got it myself. I continued to think that when I got A grades and girl B got D grades at A level.
All the others went into their interviews in 'normal' sixth-form attire. I didn't, and in retrospect it seems like it would have been a good idea, because I can't think what else would have prevented me from being awarded the post. I truly believe that what you wear and how what you wear makes you appear makes a huge difference to the way people perceive you, so do think about it (eg. if your dressing style is a bit conservative or off the wall then maybe think about changing it for just a day)! Sorry if any of this sounded arrogant and/or like a rant! xx