Hi, first of all, love the username

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Now think of your study score as coming directly from two parts. The first is an exam score - this is objectively marked by examiners who don't know you, and directly translates towards your study score and is by far the most important part of your grade, although I'm sure you already know that.
The way SACs work is actually quite simple. At the end of each unit, you're given a ranking based on your results on all the SACs combined. This is done internally and ranks every student in your cohort from 1 - 100 (if there's 100 students, etc). This ranking is then given to VCAA, and moderated by your exam results and your cohorts exam result.
The exact formula of how they do it is contentious and imo. largely speculative, so here is what you need to know:
If everyone ranked higher then you was to achieve an A+ on the exam, then your SACs would be scaled up to an A+ assuming you also got an A+ on the exam. If everyone ranked higher then you was to achieve a D+ on the exam, yet you achieved an A+, your exam result would stay the same but your SACs would most likely scale down.
Essentially what this means is that in order to achieve a good SAC mark you want the people ranked above you to do well on the exam. Anyone below you is irrelevant, but the people ranked higher then you will directly affect your SAC mark, either positively or negatively. This is why going to a school like MHS or McRob pays off in terms of SACs - there is a greater amount of people who do well at the top end, so if you go to a school with a strong cohort like MHS/McRob you do not have to even get an internal A+ average for SACs in many subjects to get an A+ from VCAA.
One more thing, another way the "strength" of your schools cohort is measured is by the overall cohorts ranking in GAT results, although that only effects scaling to a small extent as far as I'm aware.