In reply to uppersharpnose:
Stick with maths, and as others said Physics is a good one to add. If you're really dedicated for games stuff, definitely do Physics. Can you also take Applied Maths and even a subject like technical graphics (I'm not from UK so don't know what the subjects are called in your system)?
Beyond that, I'd suggest getting into Linux and learning your way around that too. You get tonnes of programming tools included on a typical Linux machine and you can learn a lot there about networking, filesystems, even device drivers if you put your mind to it. From a learning point of view, don't stick with Ubuntu, but go and get Debian or even Slackware/Arch or another big but "hardcore" distribution. Ideally you want an old machine that you can keep stripping and rebuilding (so that you're not completely blocked when your install of linux-from-source takes longer than expected!).
You could find one of the open source games that you think you could enjoy contributing to. Start lurking on their mailing lists, reading up on programming and reading their source code. Eventually contribute some bug-fixes, even features. You'll learn a lot if you do that, and I have to say that the standard of free games has improved a lot (especially as ID has opensourced some of their older 3D engines). If you did that in earnest, get a bit of a reputation, it will be like gold whenever you start to look for jobs (see earlier comment about hiring from Stackoverflow)
And really, what you're trying to do now is get in your 10k hours to become really proficient. Not much point focussing too much on what technology to get into, if you go to college it could be 10 years until you are working a job. Things change, especially tech.If you do really well at this and get very lucky indeed, have a bright idea and implement it well, you might not even need to go to college (but still go for the giggles!, better yet, you might not need to get a job!).
However, throughout your career it's a good bet there'll still be a lot of Unix, and the basic principles of programming will be there too.