Jonny, being only just 17 (4 days ago) and only in the year above you then my advice may not be the best around! However, I have spoken to many experienced pilots, and from what I know the best thing you can do is to gain as much information on flying as you possibly can. If possible try and get a PPL as this will show the airlines you are determined.
I am studying English Lang, Business Studies, IT and Popular music (the slack version of music). I am continuing all 4 onto the A-level year. The pilots that I know say as long as you have 3 C's of which none of them are "slacker" subjects (like the Pop music I do) then you should be alright. But dont forget, academics are not everything and intelligence is different to academics. I flew solo when I was 16 and should have my PPL soon. Ever since I was 13 I was reading books on flying. Someone else mentioned the Trevor Thom books, these are very good but to be honest I would only advise to read Trevor Thom 1 and possibly 2 if you arent going to train for a PPL. Trevor Thom 1 and 2 will give you a beginners understanding of flying and help you to understand the basics of flying. And dont just read them once, read them until you know every word off by heart in the books, and you know what every single page has on it. If you cannot keep reading these books over and over again then maybe aviation isn't the career for you. I find that every time I
re-read a book I learn several new things. All of these things may help in the future. I basically now have a library of books on aircraft that I have read, ranging from ATC procedures/Boeing 757 Manuals/ Instrument navigation to VOR-ILS-NDB procedures... I simply can't get enough of aviation. I am not an academic (predicted 4 C's at A-level), but I am hoping my determination will get me a career. Dont forget there are many organisations out there that will help you, for example:
www.pilotcareercentre.com is excellent.
Hope this is of some help,
Mark Wilson