School ranks helps much more to determine how hard your ATAR might be hit. Of course in all cases the rank is determined by the cohert, but if yours is top 100 they don't tend to fluctuate too much.
On a note; at least a third of my cohort is international students from countries like China,alot of them sleep in class and their ranks aren't that great yet every year at the end these people pull themselves up alot and suddenly gun down to some competitive ranks in each subject. So what I'm trying to say is...don't look at it so negatively alreadt,sure some don't care, but I doubt you talked to 40% of your cohort about ATAR and they all said they don't want one.Some of them might be studying at home instead or look like they don't want an ATAR to suddenly suprise people,etc...
If your school is ranked in the 600s...that could be a different story then.