You don't need to be super intelligent to do well in the HSC
You just have to be a grade machine
A lot of selective kids will literally not have any form of social event over a *month* before assessments/trials/whateverotherbullshitthing because they have to study
You can't learn anything new in that space of time relevant to the syllabus. All you're doing is just learning how to rehash dot points in essays. Unless of course it's 4U Math where it's a bit less about rehashing dot points and understanding mathematical concepts/approaches/techniques.
My point being is that naturally intelligent people can turn up to exams and still get mid band 5 or higher without any study. If they'd re-written essays 4000 times before that, they might be a bit better at it.