I'm from a private school. Is that a good thing if I'm trying to go in at year 10? 
Year 9 entry is much easier than year 10 in almost any case, because as long as you meet the minimum threshold in the exam (not really sure what it is, but a classmate of mine at Mac.rob got all above averages and got Principle’s Discretion for missing on the threshold by a small margin), you have a fairly good shot at getting in. As long as there are students meeting the minimum requirements in the school, there will be up to 5% places (out of the entire cohort, not just the people that tried out) guaranteed. So if the school has 200 students in a year level, the school is allocated 10 places. If 20 people (which is a decent number) sat the test from a given school, and even if they all end up meeting the minimum requirement (which doesn’t usually happen), there is a one in two chance of getting in. If you go to a single-sex school, even better, all the allocated places in my school except for one (by choice, she could have gone to Mac.rob if she wanted), went to Mac.rob. Even in competitive schools (PLC, Uni high, Balwyn tends to be very difficult though) getting into Mac.rob/MHS is still fairly achievable, since there isn’t a very high interest in most schools (and very little interest in private schools like Scotch and St. Kevins). Year 10 entry doesn’t have the luxury of guaranteed allocations by school and doing well of the test doesn’t necessarily grant you a place or an interview.
Going to a private school doesn’t make year 10 entru easier, it just severely disadvantages public school students, as a result of that MHS (it’s different at Mac.rob, where they do place value on co-curricular involvement but don’t expect significant achievement) places emphasis of significant achievement and commitment at co-curriculars. Given that the entry criteria is the type that would attract many private school students (they find the novelty in trying to get into something that is elusive and demanding compared to the year 9 entry which is just a test in most cases), going to a private school wouldn’t be an advantage. The criteria was not intended to favour students from a particular school, it’s just that the kind of achievements they demand would be far more accessible for students from schools that offers, encourages and promotes such things. Like state team try-outs in sport are promoted and encouraged in schools with extensive sporting and coaching programs, participating and being good/trying out for the state team at debating is not an opportunity offered at many schools (although there are never year 9s in the state team but a few very promising ones may make the development squad or the late stages of trials), many schools and parents can’t afford to provide kids with the opportunity to pursue performing arts and music. Obviously, not all students at private schools take advantage of such opportunities or are committed at them, but those that do so stand out. While a student from a disadvantaged background could make a case (since the application allows for that) long term disadvantage which limited their co-curricular opportunities, that they’ve done their best with what they have got, that MHS co-curricular program would allow them to extend themselves, it would be fairly difficult to pass up a state/national level sportsperson or a talented artist/musician. The personal statement that helps you get an interview, benefits students who are committed at extra-curriculars (obviously they don’t have to be state/national level or very talented), as I find that being committed at extra-curriculars allows people to find their own voice and personal development that adds a unique angle to such statements.