I have no idea how to give good estimates lol, so someone else can do that
In terms of school rank however, I can shed a little light on it.
Basically, after trials all the ranks are sent over to bostes
Then when you do the HSC exams, you get an exam mark for each exam
You also get an internal mark based on what rank you were internally, and whatever the corresponding exam mark was you get as an internal mark (rank 2nd gets the 2nd highest exam mark as internal mark)
In simpler terms, if you are ranked second internally and get an exam mark of 92 for example, then whatever was the 2nd highest exam mark say 94 will be allocated to you as an internal mark. Your final hsc mark is the average of these two, so it will be 93.
In your situation, you will basically have to be first in all your subjects to lessen the impact of your school (assuming other people will drag you down rather than pull you up lol). So for like ancient history, lets say you stay third by the end of trials. Now even if you absolutely obliterate the HSC exam and get like 99 lol, you are still going to be allocated the third highest exam mark as an internal mark. Now if third is really crap and gets like 75 your HSC mark (the one that matters) will be pulled down to 87.
If you are first however, then you will be allocated the highest exam mark as an internal, and so assuming that you are the highest - your performance is totally dependent on you. The disadvantage you face is that you don't really have the protection that someone at a higher ranked school would have if you don't do as well on an exam. For example at my school (top 10) if I am ranked first, and am expecting say 95 in physics if i flop the exams (relatively) and get 85, the highest exam mark will still probably be like 94-95, and so I can get pulled up to a 90. In that sense, having high ranks at a high rank school gives you like an insurance for the exams.
That being said, all you can really do is try your best to pull your ranks up and work hard. Worrying about most of these things is kinda pointless lol, do the best you can, and at the end of the day you will still probably get where you want to go.