From what I’ve observed at our school which is usually ranked 300-400, ATAR is never affected by anyone else’s performance if you are ranked first in a subject and also perform best in your school cohort at the HSC exam for that subject. Otherwise, if assessments are very accurate (eg. teacher’s assessment marks actually match student’s performances in the HSC exam), then in theory, the school should not affect the individual’s ATAR. Unfortunately, a lot of assessments are assignments which don’t discriminate well between students, so assessed marks are more closely bunched together and so if some of these students don’t perform as well under HSC exam conditions as their untimed, uninvigilated assignments, they can ‘drag’ the mean of exam scores down, so that individuals who scored much higher in the exams, but only slightly higher than their peers in assessments, may receive an assessment mark less than their exam mark (the exam mark is not affected). Don’t worry too much about it though. My eldest did her HSC in 2015 - the school achieved its highest ever ranking that year (around 250) and many of her friends are now very successful young adults (she’s a doctor, there are several engineers, a lawyer, as well as builders and tradespeople who have their own successful businesses). Just focus on achieving your best at both assessments and the HSC exams.