one thing they will ask for is a list of your repertoire and probably a brief bio (at least they did at wcu). i would take lessons for a while from your current teacher, or one that is an alumni because through them you will gain a bit of insight as to what the school requires also. (esp. if you take from an alumni of the school - which is easier to do privately at first). getting private lessons first is MUCH easier and that will set you up to work your repertoire list into shape (not to mention your playing). just tell them - when you audition - what your goals are (to get into whichever school) and if they could help you attain it within 2-3 years.
getting into a school at 15 really doesn't make you look any better than taking a couple more years of lessons (2-3). when you are 18 you'll fit in better anyway, imo. i could be wrong. seems that a lot is required at these schools (kind of like military school?) and you have to develop the habits that will help you - and learning to pace yourself and your class load. one thing you'll find is that finding balance is difficult. probably first, go to a nutrutionist and find out what is the best balanced diet you can eat (and don't be tempted for fast food). get into an exercise regimine (daily). i think playing piano at a high level is much like the olympics. you need a lot of energy and mental alertness.
ps i'd also get better at music theory than most of your piano student friends. take some classes at the community college over the summer in theory, form/analysis, even orchestration. if you have skill levels that are high in those areas, it shows that you have a broad range of talents. save your best work in a folder. one class that is great fun, too, is conducting. it teaches you a lot about interpretation.
also, whenever your schedule allows, print up a program of your choice and perform it for the public in different places (museum, garden, church, retirement home, mall, restaurant) and then you'll have some proof of your performance abilities (that you don't get so nervous in public).