Okay, so unless there's been some confusion, your sac marks don't actually matter - just your ranking.
If everyone in your cohort fails the sacs, and then they do exceptionally well in the finals, your Sac marks are standardized to match the exam score, and you can still get an A+ for sacs. Conversely, everyone in your class might be get high 90s because the tests are really easy, and then come exam time, under-perform, and the Sac marks drop below B.
Seeing as VCAA ends up standardising everything, you don't need to worry about your marks - just your relative position in the class, as your Sac marks are judged based on the performance of the #1 rank in your cohort.