https://en.wikipedia.org/wiki/Equality_of_outcome VS
https://en.wikipedia.org/wiki/Equality_of_opportunity <--- Important here.
I know some people who have gone to absolutely terrible schools (eg. parents showing up with baseball bats during lunch because of what a kid did) and got in the 90s. We have inflated standards here but anything over 70 or 80 is very well done i think, at least in terms of actually getting you into a university and getting you a degree.
Education is one of the strongest promotors of social mobility. Myself and almost all of my friends had parents from working class backgrounds (no uni degrees here folks) and a fair few didn't even finish highschool. For us, even finishing highschool or getting into any university is an achievement in itself. I think we've progressed in leaps and bounds.
30 or 40 years ago, only about 3% of people had a uni degree, now its hovering around 30%. We're clearly doing something right.
I think in terms of once you get into university, its pretty equal. Even if your atar score is somehow wholly to blame on your schools (not likely) failings, there are still pathways to get into uni and get where you want. Hell, if you work hard enough you can transfer into the GO8 from pretty much any uni or into law or something insane like that.
TL;DR Good school helps, everyone should be given an equal chance but everyone shouldn't be given an equal outcome just because they feel entitled or want to pin all their problems on a horrible school. To be honest, if you wanted to get into uni, it isn't terribly hard. I think almost all those that wanted to and put their head in the game, did. Of course there are kids that didn't get into uni but plenty of those never wanted to go or needed to go in the first place.