School rank and student rank are not taken into consideration by UAC.
The marks that UAC uses are your raw HSC exam mark (your performance in the state) and your moderated assessment mark (your rank in the state). These marks are relative to the state - ATAR is relative to the state. That's all they need to determine your ATAR.
The combination of these 2 raw marks forms your raw HSC mark. UAC scales this mark, then adds your best 10 units to form your aggregate. You are then ranked among all other students and you get a rank, i.e. your ATAR.
The ATAR calculators use your aligned HSC marks. UAC releases tables that indicate the HSC to scaled mark conversion. By putting in your HSC marks, the calculators add up the scaled marks giving you your aggregate and thus ATAR.
They are accurate because they use the data that UAC releases. This means if I put in my marks in the calculator that uses the 2011 data, then it won't be an estimate, it will be the ATAR that I achieved (although since UAC doesn't provide enough data points, it can still be off by a bit).