Sorry, this is what I was trying to articulate: My school offers the IB and due to this, our rankings are quite low. When you say your school would be top 20, I think you're misinterpreting the ranking system. Ranks are in place so that the BOS can determine the quality of the HSC candidature. So when you say that theoretically it would be top 20 with IB, I think you're assessing the quality of teaching.
It's really difficult to explain. You either go to Monte or MLC, both have great scoring students. You'll do well.