Of all the subjects I studied at O level, A level and degree level I reckon the most useful ones have been (both in my working life and outside) Maths and Physics. No doubt about it!
Of course a good broad all round education will always be a big help, whatever you do. Make the most of it while you can, and while you're young enough for it to be free..... It's much more difficult to go back to it later when you have other responsibilities like children of your own and a job!
Good luck with the flying!
