PikaPika
Hi there,
I feel that you may have misinterpreted me. If not, I apologize.
Of course, our SAC rankings will place us inside of our school cohort. However, it is not 'just' this, as it may have a great impact on your study score.
Your grades at school don't mean anything (as far as VCAA is concerned). Instead, they want your school to determine, with sufficient precision, who the best students in the grade are. As such, they moderate your SAC scores, to accommodate to the difficulty of each respective school's SACs.
Ultimately you are correct. Teachers always tell us to study independently for the SACs, but collaborate for the exam, to ensure that our SAC scores are higher.