At the end of the day a school is only as good as it's students and teachers are. It just so happens that there's a higher concentration of people who are academically oriented in said school. Doesn't mean anything else- you can still do poorly in the HSC even in the best school, and you can still do well even in the worst school.
But then one has to ask... how much does the HSC really matter in the scheme of things (i.e. once you've gotten your ATAR and into uni). At the end of the day those who are considered 'successful', regardless of how you chose to measure it, very rarely attribute the success to the school they went to because it's the person as an individual who makes the success. Not the school.