they don't just look at your marks, they look at your school's atar history. obviously some schools mark quite harsh internally so that the final hsc marks get moderated and become super high, and unis have to take that into consideration. this will be a blessing for some, and a curse for others. i know someone who is an absolute genius and has already done his math ext 1, ext 2 and physics hsc and got a band 6/e4 for all of them (when he was in year 10!!! the only reason he's waited until now to do his other 4 hsc subjects was because he didn't want to leave his friends lol) - but he didn't get his top ANU early entry preference because his school's atar history brought him down. For me, I was expecting to get around 94 for my selection rank but because my school does well in the actual exams, i got bumped up to a 98.55 (not that i needed it - my course only needs an 85 lol). So keep this in mind! if your school has done well in the past, you have absolutely nothing to worry about. if your school is kinda bad, you're gonna have to be really good if you're trying to get a super high selection rank.