Listen, it's really up to you. If you want to excel and get band 6's, I suggest you start now. However, if you just want to do average, I guess you could start whenever or never. Personally,
- Minimal study for english (text types, and maybe a practice paper)
- No study for computing skills
- A few past papers for Maths
- A few past papers for Science (Pretty common knowledge)
- I would though study History and Geography (they seem like the pretty hard ones from the exams).
Take advantage of any revision classes at school. I wouldn't overstress, its on SC. If you're going in Years 11 & 12, SC isn't really important. Seriously, just study now, its only one week. You'll probably boost your marks by 20% and be heading for band 5's in all your subjects if you do so. That's my suggestion. A bit of study won't hurt.