Yes. you can get a top over 90 but even higher the higher your school ranks the better because at the end of the day you want to achieve the highest possible atar to get into you course. right?
so, here is a plan that you can use to tackle the HSC:
for Maths: Make formula sheets for all your topics and practice as many questions. repeat each day till the day of the exam. bam your ace maths.
for English standard. make essays and write your creative. check with you teacher for feedback.
For economics, physics, legal are are content based subjects. They can not ask you any think outside the syllabus. right, so make your own notes by using the syllabus (From BOS), then each day spend a time to review your notes. if you do this repeatedly, your ranks will rise up so quickly at school. I am trying to help you as much as I can because I was in your situation once but after I moved to James ruse, I started to work extremely hard in year 11 and now I am in top 5 for everything. ready to ace all the exams and get state rankings.
So follow those steps above and let me know if it really help you lol.
good luck with everything and remember your reward will be given at the end, if worked extremely hard enough to achieve your goal.