I worked ahead of the rest of the class class for the whole Chemistry Prelim course because I couldn't stand going so slowly (relatively speaking). In a way, it really helped because I learnt things twice - once by myself, and once again when the rest of the class got to it. We're onto the HSC course now, and I'm starting to work ahead again. If you can, talk with a teacher about what you're doing and make sure you're staying on track. Like Dreamerish said, it's really good to move ahead if you can - I think I would have ended up slitting my wrists had I not done so for the Prelim, I was that bored.
Oh, and pay attention to the Energy chapter, especially the parts on alkanes and alkenes. Your first HSC topic is directly tied into those. Just because you work ahead is no reason not to be thorough.
I_F