personally speaking, i attend a school which in my HSC year was ranked around the 150's and i managed to come out with an ATAR just slightly above 95. I remember also asking the same question u did before entering the HSC (since my school is also partially selective- ish), with a major concern on the mainstream students weighing me down.
However, what you need to understand as i did at the time; is that going to a lower ranked school does not mean you will get a bad ATAR, it just simply implies a higher importance for you to rank higher internally, specially if you consider your cohort dead weight.
Have the mentality that if you are the best out of your entire school, then you should rank 1st in all your subjects without trouble, and i can promise you that if you rank first in all your subjects and continue your efforts throughout the external examinations, you will definitely get an ATAR above 95... Maybe even above 98 if you do extremely well externally + decent scaling subjects.
Happy studying!