Hi,
That's alright, I didn't even learn what a study score was for a long time so there's certainly no pressure to already know about this stuff.
In terms of your what your grades mean, your average SAC score doesn't really mean anything to VCAA. Basically, since the schools set their own SACs VCAA uses the exam to see how "good" your cohort is, and then creates the SAC scores based on that + the ranks of people in the cohort. By rank, we just mean if you got the highest score (in your schools' classes) in the SACs, 2nd highest, 3rd highest etc.
If you are rank 1, then whatever the best exam score from your school is, that will become your SAC score, so don't stress too much about if your school is underepresented or not.
eg. if you are in the top quarter of your school, and everyone in the top 25% gets an A+ on the exam, your SAC score will be A+ (even though your average was 71.3)
We have a section of the forums called the
Victorian Technical Score Discussion which provides some good explanations about all of this if you want more info (definitely feel free to keep asking questions!)
Hope this helps!