I went to a top 10 school too, and despite getting trash ranks, I did very well in externals and got 98+ atar! The school's atar estimation even had the audacity to give me estimates of 92 and 95.5...hahahahaha
For reference, my final approx ranks were English 90/160, Chem 30/80, 3u maths 60/120, 4u maths 60/80 (I did a really uncommon subject as well, which scales about as well as 2u maths, and my rank was 7/10)
Best of luck!
Also don't listen to the trolls on here lol. Once I asked for an atar estimate and a troll gave me 96...still higher than my school estimates HAHAHAH