^
Not to mention the entrance exams and costs it takes to get any child into those selective-entry schools. I guess people who care about their education will excel, irregardless of which selective or public school they attend.
MHS = Free

They do ask for you to pay for fees (around the 2k mark), but they can't force you, it's a public school.
They also receive millions of dollars in donations from old boys which allow them to keep up with and often surpass private school standards.
The whole idea of MHS is that money doesn't and shouldn't buy education, everyone should have access to resources to excel etc.
It's probably a 40-60 split between students who come from public-private educations previously.
Plenty of kids at MHS don't care about VCE, they were good enough to get into the school, and should be allowed to stay as MHS and school is about 'more than just marks', and this view is shared by all people with a link to the school (apart from Asian parents?)
And as Mao said, so many kids don't live up to their potential and some clearly exceed how intelligent they really are with their ENTERs. For example plenty of students at MHS score say 90, if they were at your standard high school they would score say a 80, happens heaps. The environment is such, that it produces high scores consistently.