I am actually in charge of attendance at a large Secondary school in the UK. It is the govenors who decide the ruling on allowing (or not as the case may be) pupils time off for holidays in term time. Our school allows pupils in years 7, 8 & 9 upto 10 days off providing their attendance record is good (95% or above). No holidays are authorised at all for pupils in years 10 & 11.
However...............even if the leave is NOT authorised does not mean that you cannot take her out anyway. The leave would be coded as unauthorised holiday which is nowhere near as bad as unauthorise absence. EWO (Education Welfare Officer) would only ever be called in if the pupil's absence is seriously low - a 2 week holiday would not put you anywhere near that zone!
My advice is...DO IT! The experience she will have skiing on much less busy slopes and the fact that you will have loads more dosh to use on fuel for ribbing next season will far outweigh anything else
With regards to the letter you write - the sooner you put in the request, the better it will be regarded and you can always use restrictions with one parent's work or family get together as an excuse. But generally the reason wont really make any difference, it will be the schools own guidelines that will used to make the decision.
If it makes you feel any better, we have AT LEAST 3 or 4 pupils who take unauthorised holiday every month and none of those had anything bad happen to them.
Until I worked in a school, we took our two girls out every year to go skiing and it has done them no harm whatsoever other than turned them into much better skiiers than us
Have a great time