sorry one more question, i might have got this all wrong, but if i do better than rank 1 on the end of year exam does that mean i will get their sac mark??? so confused
unfortunately , no. eg. your number three in your cohort, you kill your exam, but rank 1 and 2 dont do well, you will get their exam score as your 'new' sac score, but you're exam score will stay the same. Sacs only ever change. If you do really well in your exam, and you aren't rank 1 and not in a really academically strong school, you will be 'limited' per se.
essentially, aim for top of the class and ensure you can do well in the exam! If at the end you're not, you may be dragged up or down based on how well your whole 'group' does
edit: may have confused you and anyone else reading this, either PM me or read a post by Hamo94 that talks about sac scores and moderation , i'd link it but i can't find it :p