Hi Kid,
I can't believe you haven't joined the Air Cadets yet. I joined at 16 and left at 18 and in those two years I managed about 40 hours powered flying experience (Chipmunk, Bulldog, Jetstream, Dominie, Sea king, Firefly etc.) and also earned various Gliding badges.
Despite the fact that arrogant nerds wearing Corporal/Sergeant badges and abusing their authority will get on your tits, the chance to fly for free cannot be turned down. Showing such enthusiasm for flying will prove to employers of the future that you are serious.
As for school; Maths, Science and English are very important for flying but so is the stuff you enjoy. Try to find a balance and if you fancy the RAF, try to keep fit and maybe join your footy/rugby team etc.
There has also been a mention of work experience in one of these replies, I managed to do mine at RAF Cranwell twice. The RAF are more than willing to take work experience kids but it takes a while to process the admin so if you fancy that, try to sort it out as early as possible.
I wish I was as level headed at 13 as you sound, you seem to have the sense to plan things well, good luck.