Thank you for all your replies, this will all be of much help to me. Yes, proms in England are black-tie although somebody in the year above mine dressed up as the Stig from Top Gear last prom. One of my friends did also wear a kilt to prom.

Firstly, how old are you? Secondly, how often do you think you will be wearing a kilt after the prom (realistically). I live in Norfolk and the opportunities for kilt wearing are few and far between! I have to create the events to wear a kilt.
I am 15 and I think that I might wear them now and again, probably just for fun when I visit my friends abroad. :P

A full kilt outfit could cost a few quid short of a £1,000 That would be a formal outfit, not something to visit family or a night out with the lads. You would need other jackets and such for that.
I don't really do things such as a 'night out with the lads', but if it's that expensive do you recommend I just rent one?

I'm not overly sure if I'm still going with the girl. One of her friends told me that she won't go with me if I wear a kilt 'because she wants me to look smart'. Even though I tried to explain that full Highland Dress is considered to be very formal. More formal than most people there will wear.