However, in my opinion studying ahead becomes an issue when you study ahead just for the sake of it and for no real intended purpose.
I absolutely agree with this. Having the dedication to study ahead is no doubt an arduous task, and I've seen people studying ahead like a damn chore. I think this is counter-productive.
You will start to lose focus, start to despise the subject, and might possibly obtain worse results.
However, if you are motivated, feel like working ahead because you enjoy it then go ahead. I just don't think it's necessary if you feel that is mandatory, and feel forced to do it.
I'd recommend working ahead in the summer holidays, by opening your book when you feel like it, and probably doing the first couple of chapters.
You'll enter the class (probably a month or two ahead), and if you work consistently throughout the year, you can finish the course a month or two ahead of your class, and start prepping for exams. (Another thing a lot of people do is rush the course and finish halfway through the year, I really don't recommend it. I think you should cover the course the same rate as taught in school (but given you started earlier, you will finish earlier)
It really depends on the person but if I commenced the year whilst completing the course what would I do for the whole year?
I'd most likely just slack off, waste time, and forget everything I've learnt a year ago.
So if you are up to it, it takes serious dedication and make sure you revise and review old topics constantly. You'll be surprised how quickly you will forget them if you don't revisit them.