It is theoretically possible to do well at any school despite rank and badly at any school despite rank. The main reason for estimates you need school rank is because as you can imagine, coming 10th in maths ext 1 at ruse and 10th in maths at a school ranked 400 is quite different, as the cohorts are of different quality. Of course it is true, school rankings can change from year to year, especially with lower ranked schools (the top 10 does not change hugely from year to year), but seeing as we can't predict the future, using past rankings is the best we can do despite it not being perfect.
So if someone is doing an estimate, they would look at your class ranks, schools ranks and any other information you give them and for example if you went to ruse and you were coming 1st in maths, they would know that you were a very strong contender for a state rank, and would estimate a mark in the high 90s for that subject. But if you were coming 15th in maths at a school ranked 200, they might give you a mark in the 80s or 70s because less people from that school typically get Band 6s. As a general rule estimates are harder the lower ranked the school, due to the variation in cohort from year to year