From my personal experience as a former student at Scotch College, the disparities between the four institutions are usually nuances, albeit substantial ones; whether those be with regard to general aura and school tradition, the scope of extracurricular opportunities, academic rigour, or their respective sporting cultures. By that token, it is probably worth separating this response into those four sub-sections.

1. Of the four schools, Scotch College and Melbourne Grammar are the oldest and their traditions perhaps the most prominent. While I cannot speak in specifics for Melbourne Grammar, at Scotch there is a whole school assembly three times a week, during which there is always a sermon, doxology, hymn and principal’s address- testifying on a small scale to the pride and earnestness invested even in weekly routines.
   Equally, Scotch and Melbourne Grammar have the most proactive and well-connected alumni base, and have consistently given rise to eminent politicians, business leaders, scientists, authors and legal figures. From the perspective of heritage and reputation, Scotch and Melbourne Grammar are universally considered the most established, perhaps by virtue of their age and Anglo-Saxon legacy. While prestige seems sometimes to be inextricably linked to a lack of cultural multiplicity, over the course of my schooling I have found that Scotch hosts the most racial and ethnic diversity of the four schools. That said, Scotch seems to remain the most conservative school in that group.
   Xavier, as a Jesuit school, is inherently much more fervently preoccupied with community service than the other three schools. Ironically, this is in tension with their somewhat chauvinistic reputation as a school (whether justified or not).
   Meanwhile, St Kevin’s is the least expensive of the four schools and the most socio-economically diverse.
   As Catholic schools, the perception is that Xavier and St Kevin’s are more ardently religious, though I am not sure that is necessarily true.

2. From the perspective of extracurricular opportunities and facilities, Scotch is difficult to parallel. The music centre (James Forbes Academy) is easily the most comprehensive of the four schools and has earned international acclaim (overtly translating into the quality of a music program which has cultivated many lauded musicians), while the sporting fields and facilities are immaculate. Furthermore, the recently-constructed Sir Zelman Cohen Science Centre offers the best science and technology resources in the state.
   Any and every competition and extracurricular activity with adequate interest is happily funded by the school, as well as training programs so that students not only participate in these activities, but achieve at the highest level. Nevertheless, all four schools are very much driven by a desire to provide a holistic education- Xavier is probably the least compelling in terms of its involvement and success in extracurriculars such as debating, public speaking, and other external competitions, while all three of St Kevin’s, Scotch and Melbourne Grammar figure dominantly.
   Scotch rates particularly well in mathematics and science competitions, as does Melbourne Grammar in the humanities.

3. Academically, it is important to note that none of these four schools are selective, and usually have a median ATAR around the 90 mark. Therefore it is more apt to look at the top end of each school to properly compare these schools academically.
   Scotch and Melbourne Grammar almost always have multiple 99.95 recipients, while Xavier and St Kevin’s are more erratic and their success contingent on their cohort. In 2016, Scotch College had 12 students receive 99.90 and above; significantly more than any other school in the state. Of the four schools, it is relatively safe to say that Scotch boasts the most well rounded academic across a myriad of large subject areas, from Latin to Specialist Mathematics, Literature to Physics, etc…
   That said, in recent times St Kevin’s has been particularly impressive in English- with six perfect scores in 2016 despite a comparatively small cohort- as well as in P.E, and History. Melbourne Grammar is similarly strong in its humanities, namely across Philosophy, Literature and English.
   None of the four schools hold particular sway in the Business and Commerce oriented subjects.
   In terms of acceptance into the world’s leading universities at an undergraduate and postgraduate level (Oxbridge and Ivy-League), Scotch and Melbourne Grammar once again lead the way both amidst these four schools, and across the state.

4. The discrepancies in sport between the schools are relatively trivial, to be completely honest.
   The only sport in which there is a distinct chasm is rowing, which has been controlled by Scotch- and at times Melbourne Grammar- for a while now. Both those schools bear quite a bit of weight not only in APS rowing, but on the national scene.
   The first AFL match ever was contested between Scotch and Melbourne Grammar, and as such the contest between these schools is fierce. However, Xavier and St Kevin’s are currently as, if not sometimes more, competitive in many of the conventional Australian school sports- AFL, cricket, soccer, tennis, basketball, swimming and athletics. The sporting success of these four schools oscillates on a regular basis.

Hopefully this gives some valuable insight into what are four brilliant institutions.

In all honesty, I think this comparison can be categorised into two seperate leagues.

Melbourne Grammar School vs Scotch College

and

St Kevin’s vs Xavier

The former are in an entire league of their own, whilst the other two (although admittedly, are great schools), don’t compare.

I am currently a student at Melbourne Grammar and at one point was debating whether I should attend Melbourne Grammar or Scotch College. All four schools are very accomplished and their differences are often only minor nuances, but I can only speak for my experience at Melbourne Grammar and my experience in choosing whether to attend Scotch or Melbourne Grammar.

I believe that the difference (in terms of academics, extracurriculars, etc.) between Melbourne Grammar and Scotch College is so minor that personal affinity to a school is more important when deciding which to attend. There are obviously certain differences between each school (as explained in detail in another reply), but quality of education and opportunity for extracurriculars are the same. The success of a student at either school ultimately comes down to how proactive he is in studying/seeking out those opportunities.

I ultimately chose to attend Melbourne Grammar over Scotch College because of my impression and my initial affinity for the school. Travel time from home, school fees, location, diversity of student body, academic success, and even the school landscape were all reasons which contributed to my final decision. To me, finding the difference between Melbourne Grammar and Scotch College would be like splitting hairs - both schools offer fantastic opportunities that reaching a decision, at least with my experience when choosing which school, came down to my impression of the schools.

The previous replies are very biased towards the Scotch and Melbourne Grammar parties.
I went to none of the 4 schools but know many ex students from all.
I’d say the Scotch and Melbourne boys are a bit more preppy than Xavier and St. Kevins. Lots of relying on Mum and Dad still at the age of 26, and took a bit of time for them to truly welcome me, great blokes nonetheles.

I’d say Xaviers Old Boys associations are probably the most i’ve been impressed with. Those guys are always at those sort of events, and have fantastic networking.
St. Kevins not so much in my opinion.

Melbourne Grammar, Scotch, Geelong Grammar are Victorias oldest schools and therefore perceived as the most prestigious based on their White Anglo-Saxon Protestant history. The first two are high achieving schools academically but Geelong Grammar is not known to be particularly high performing.

St Kevin’s and Xavier are both Catholic, hence for some, might not be perceived as having the ‘cache’ of their Protestant contemporaries although St Kevin’s in particular has a very good reputation and is known to be a very high achieving school (The top 3 schools are always Melbourne Grammar, St Kevin’s, Scotch) not in any particular order as there is usually a very slim margin between them and each year the order is different and largely irrelevant as the results from those three schools are absolutely outstanding.

Like Geelong Grammar, Xavier is not known for its academic results although Geelong Grammar is much much more expensive. You can pay a lot of money and not necessarily get great academic results (All these school offer excellent sports and co-curricular, so its not always All about Academia)

Its not easy to obtain a place in any of these schools. St Kevin’s in particular would be almost impossible. Id suggest the most likely school in which a place might be attainable would be Xavier. Its a large school on impressive grounds and being Catholic, the fees are not quite as steep as Melbourne Grammar and Scotch.