Your ATAR is not a function of the school you go to nor is it a function of the amount and time you spend on tuition.
The truth is, much of what happens in year 12 is a product of your own effort, time and work ethic and I can assure you it doesn't matter whether you go to the best school in the world or you spend thousands of dollars on tuition, unless you make the most of it, and the only way you can make the most of it is to put in individual time and effort looking at what you need to know and how best to learn then, then using the resources you have around you.
In the end, the fact that you go to a good school and the fact that you get private tuition are just resources that you have at your disposal. Those who don't go to a good school or who don't get private tuition can find different resources in different ways - such as books, study groups...etc. It doesn't mean that just because you go to a bad school you are doomed, neither does it mean that if you go to a good school you will succeed.
Just to put it into perspective, a student from St. Albans Secondary got 99.95 last year, a school which is quite low on the food chain, whereas I'm sure there are kids from MHS and other top tier private schools getting in the 60s.
Hahaha :p But the same could be said about Melbourne schools, almost everyone at GWSC has tutors for the majority of their subjects.
Not true - I didn't get tutored, apart from a few sessions in English and I know a lot of other people I can name that didn't get tuition either.