Don't worry about it.... if you're bad at Geo, just don't do it in years 11-12. Don't worry about studying for it, as it's only the SC and pretty much means nothing. It only really means something if you're going out into the world of work (in which case they're only really going to care that you've actually got the certificate, and have decent ability in English/Maths... IF they care at all)
I was terrible at Maths, and was okay about only getting a 72 in the SC - because I was not doing it in Year 11 and 12.
Remember, just think about what's going to benefit you in your senior years. I think the most important part about year 10 is making sure you pick the right subjects for year 11 - 12.
The SC is hard to study for in the first place, because it's mostly skills based, and the only time you should worry at all about your marks, is if your school let's students into certain subjects based on their SC marks. Eg- You can only do Advanced English if you got a Band 5-6 in SC English. You can only do Extension Maths if you got a band 5-6 in SC Maths.
Really, if you're going on to do your HSC, and you don't plan on studying Geography - don't bother stressing about it. As soon as the SC is over you will never have to do Geography again in your life - so what's the point in worrying about it?
Keep cool, and good luck in the SC. But remember - don't let it worry you. It's really not very important, and is pretty much designed so that people leaving school in year 10 can have some form of qualification. It is aimed at the average students, and is never too difficult or hard to cope with.