SAC marks do not really contribute to your ATAR as much as the final exams do because the SACs are standardised after the final exam. Your school cohort is compared to the schools in the state based on their exam marks as a way of determining the standard of the school's students. Your scores are adjusted based on your own marks and the marks of other students in your school and of course the state. Your ranking within your school is more important than your SAC marks. When the SACs are standardised, your ranking within your school stays the same.
Is your school ranked high? Find out
here.