Might I come out and suggest that going to private schools for "better" academics isn't really the point? Whilst it's true that by nature private schools will often "buy" out better teachers/resources/etc., many parents recognise that academic excellence is probably enforced more rigorously at schools such as MHS or MacRob, where everyone pretty much pressures each other into doing well (and given from what I've seen, probably to a harsher extent than the competitiveness at private schools).
The reason so many parents send their kids to private schools is because of the "human" education such schools provide: I don't intend to sound elitist, because I firmly believe that it's the student that chooses the school, rather than vice-versa (ie. a lot of kids will get a lot more out of a public school education), but the focus of private schools is to foster integrity and well-roundedness within individuals. Things like "traditional" house systems, compulsary sport/community service, well-funded music facilities etc. are there to allow students to build upon their non-academic side, and I think it's no coincidence that a lot of high-profile individuals come out of such a system.
With that said, I'd like to re-iterate that I don't think this makes private schools intrinsically better. Obviously it depends a lot on what kind of person you are, since if you want to focus on academics or somebody who wants to go into research, say, then I can't see the private school system being all that worth it. Hell, even in terms of co-currics, there are alternatives which can work better for some people - VCA for Music/Theatre, a LOT of schools (or even just a sporting club...) for sports, etc. What I'd just like you all to realise though is that there IS a point to private schools, and I for one am personally quite happy that I was given the privelege of attending one.