You can't. If it's boring, it's boring, and since you need to do so many different subjects at GCSE, most of what you study will bore you to death. There's no easy solution, you just have to stick with it and work according to how much you get done as opposed to how long you study (to account for lack of concentration/daydreaming).
For the record I got straight A*s (so would be considered an 'A* student') but I experienced the same feelings as you, probably worse. At least so much that I always ended up self-studying/cramming everything last minute, but when I did I think simply the stress of it being too late motivated me to concentrate and work extremely hard. I don't advise this though, just work hard consistently throughout the year and set targets according to how much you get done, not how long you study.