Well the Assessment Mark your school sent in to BOS is not going to be reported on your HSC.
The Moderated Assessment Mark which is then averaged with your Aligned Exam Mark, determines your HSC Mark.
Class ranking determines the way in which the marks obtained by your cohort in this year's chem exam are redistributed across students. So, if you've come first, for example, then you will get the highest aligned exam mark achieved by your cohort as your moderated assessment mark.
Relative gaps are maintained between students when exam marks are redistributed to form moderated assessment marks. So, if you have a 'HIGH' assessment mark, which perhaps makes a large gap between you and others in the group, then you are in a good position.
If your classmates performed poorly, then that might drag down your moderated assessment mark, since there is less in the pool of marks to redistribute. The exception is if you've come first, then regardless, you will attain the highest aligned exam mark as your moderated assessment mark.
So a high assessment mark, sent in by your school, does not always guarantee a good HSC Mark. It's your ranking that counts.