I have been to both a fairly elite private school and MHS.
I think the environment of MHS helps those who are smart but initially don't study as hard as they could to fulfil their potential. At their old school they are a big fish in a small pond, with minimum effort they top the class. Or, they are satisfied with being average, with no work at all.
Whereas when this student comes to MHS, they are with other high-achievers, and the friendly competition often stirs them to focus and put more effort into their studies. Marks at MHS are almost a status symbol within some groups.
Plus, when the majority of the students around you are working hard, hitting up State Library every second day, you can easily get pulled into this, thereby giving your best shot at VCE. Moreover, the academic guns at MHS help the lesser-achieving students very often. I learnt so much from my mates in every single subject I did.
At MHS there is definitely an aspect of "pulling up" the lesser students, through the work ethic, the help of having high-achievers around you, and the more competitive nature of the classroom when compared to other schools, including the elite private schools.