Had my 6th year prom about 8 years ago (God thats depressing) and like you opted for the Kilt (as did 90% of the year)
Don't wear boxers, traditional not to and why wear a kilt unless you are following a tradition. One point of warning though, slightly inebriated I plonked myself on a bench legs spread wide as I normally sit. Unfortunately I forgot to tuck my kilt down in the middle and ended up with pictures of me + kilt + tackle exposed around the school. Lesson learned.
As for what to buy, something special like a necklace with matching bracelet and ear-rings, corsage etc. Don't make it big and cumbersome like a huge bunch of flowers - she will have no where to put one.
Have a cracking night out - where are you going?
Oh and is it St Margarets, Inveralmond, James Young, West Calder or Deans?