Are you serious?
Go and get that mark.
Just going to respectfully disagree on this. VCE's not all about grades, it's about the friendships you make and the unforgettable experiences you have.
It's not worth potentially burning bridges with your friend (esp if you cost him the mark) for nothing at all. Best case scenario, you get that SAC mark (which is easily obtainable in future SACs if you just put an extra 20 minutes to study per day) and an extra rank, which could be a study score or two at most at the end of the year.
I can only see two scenarios where that might be important (to a significant degree)
-You're 0.05 off your course and that study score gives you an entrance
To this, you should be pretty much preparing with enough leeway to get into your course, instead of trying to just 'hit' the mark
-You get 99.85 and not 99.90 (and thus missing out with 50k in scholarship money)
If you think that this study score would make a huge impact on your ability to get chancellors, then by all means go for it, but if this wasn't the goal that you were trying to go for, then personally I don't see any huge reason to go and scrape SAC marks.
Just scratch it down to unfair marking and let this motivate you to do better in your future SACs.