Knowing your rank is absolutely essential to having any sort of idea of how you are going and what study scores you will end up with. A raw SAC score without any idea of your ranking is meaningless. With a rank and some idea of your cohort's strength, you can calculate your GA1 and GA2 scores relatively accurately by looking at the grade distributions from previous years.
Knowing your rank leads to better outcomes for everyone. For instance, if you are aiming for a 40, and you find that you are ranked in the middle of your cohort at an average school, then that is going to be a problem and you will need to work harder. Similiarily, if you are stressed about English and aiming for a 30, but you find that you are at the top of your cohort, then that stress can be relieved and you know that you don't need to work as hard.
I suggest you press your teachers to reveal your rank - hiding it from you sabotages your VCE