your actual school rank has no bearing on your atar/hsc marks.
But your classes performance does.. this is because everyone's ASSESSMENT mark is based on two things..
1) your rank and the size of the gaps between each rank
2) the performance of the cohort in the external HSC Exams.
The place where your cohort affects your marks is in your assessment mark.. this is because the MEAN (average) of the HSC marks becomes the MEAN (average) of the assessment marks. So if your school/class does well in the external HSC exams it could pull you up but if they do bad it could pull you down. Because your from a top 20 school, I'd expect your peers to do well in the HSC exams and "pull you up" if needs be.
But in terms of "pulling up" and "pulling down" it also comes down to your OWN performance in the HSC exams. Because everyone keeps their own HSC exam mark, the only mark which is moderated/affected is the assessment mark.
You need to aim for high ranks to ensure that you are not pulled down, no matter how highly ranked your school is.