Nasher what 'future captain' has said is very good advice. Pick subjects that you enjoy. You'll do a lot better in them and will find it easier to work harder with subjects you enjoy learning about.
I did English A level because at the time I thought that airlines would prefer this solid subject compared to something like History of Art. I spent two years of been totally bored reading and writing terrible essays on Andrew Marvel. I simply didn't have the dedication to the subject and in the end just scrapped a C grade. Luckily throughout all my flight training it never effected me but I wished I had done another A level and probaly have gained a better grade.
As 'future captain' said try to join in team activities. E.g rowing, football team basically anything which shows your used to working as a team or key word 'crew'.
I would especially recommend joining the RAF CCF. This will get you working in team activities, shoot a few rifles, fly in some interesting aircraft ( I once flew in the back of a Hercules) and teach you leadership skills. Also there are still a lot of airline pilots and instructors who are ex RAF who maybe giving you a future interview so with the RAF CCF history you have an 'empathy point'. It helped me out in a recent successful interview. I talked about my time in the RAF CCF for about 20 mins but my GCSE and A levels weren't mentioned once by anyone.
Don't forget to have fun at school and best of luck.
Mooneyboy