technically this is true - in practice it is far more difficult due to the comparative scaling and manipulating results to fit the bell curve. but back to the original question on selective high schools --> no, you don't have to be at one to get a good UAI, but it does help. Generally speaking, selective schools are attract a higher calibre of teachers (once again, let me say, I'm generalising here, there are always exceptions) and, most importantly, a higher calibre of students. now, when I say higher calibre of students I am not trying to sound snobbish and say that people from selective schools are smarter than everyone else - that's not what I meant. What I mean is that there's a greater proportion of students who are at school because they want to be: there's a higher level of dedication. This creates a better academic environment for all students. It is, after all, a proven fact that people perform better when they are competing against people of the same or higher abilities than themselves - nothing like a bit of incentive to help you perform!
Now, please understand, I'm not trying to say that you can't get a fantastic UAI at a traditional public high school. However, contrast the person who said that they got 99.35 with 7 other people in their year getting over 90. now that's great, however contrast that to my old selective high school - in 2004 (my grad yr) we had roughy 160 people in our year and out of that 160 over 90% of the students got a UAI of over 85 with most getting over 90. our top mark was 99.99 and we had 6 others who got in the 99.9+ range. my school, by the way, whilst selective, is not by far one of the top 5 selective NSW schools, so just imagine the results from places like James Ruse and the private colleges like Kings. Nature vs Nurture: your school can't help what nature has provided, it can greatly improve the nurture aspect though