Hey man, sorry for the late response.
I went to a school ranked around 180-190 and ended up with a 99.80 ATAR. I struggled big-time with Mathematics Advanced, averaging 70% in both exams in Y11, 10% below average, (these exams were at a public school and were not difficult) and ended up achieving 94 as my HSC Mark, as well as Rank 1, due to sheer determination and commitment to the subject. Not to mention, I managed to recover my Modern History rank on four separate occasions, moving from 10th to 3rd to 2nd to 1st after tanking the first assessment (5% below average mark).
As dumb as it sounds, you can genuinely do anything you put your mind to. I was determined to get a 90 ATAR before Year 12 began due to my poor performance in Year 11 but as the year went on, as I studied and got on the grind each night, it amounted to a goal of 99.95 by the end of the year. My ranks were pretty tragic in Year 11 (i.e. top 25% at a public school equates to bottom 5% at top selective school). Whilst I didn't achieve my final ATAR goal, achieving my ATAR at a middle-low ranked school is pretty solid and is due to consistency and not giving up too early on. There is always room for improvement, even if you achieve 100% in an assessment task. There is always a second opportunity to showcase your knowledge. One assessment or exam is certainly not the be-all and end-all.
The HSC is surprisingly psychological. Adopt a strong mentality early on (growth mindset) and you will soar throughout the year. Don't compare yourself to others, just focus on yourself and continue working hard and it'll pay off in the end. I always compared myself to my friends from selective schools throughout Year 12 but ended up beating all of them ATAR-wise, so it's kinda funny how that worked out.