Not necessarily - there will usually be a few people from those schools who get over 90. The only reason the school rank matters is because your internal assessment mark takes into account that you are competing against different students at a top school to one of the lower schools. However, there are people who get brilliant marks going to a poorly ranked school. You'd probably want to be top ten in most subjects to get over 90 though, while someone at a selective school (competing against more acadamic students) would only need to ranked around the middle.
f3nr15 - we should get a letter soon. Internet is at
http://studentsonline.nsw.edu.au and our school ranks are there from the end of the hsc exams so we can check they're correct.