My school is ranked about 530ish and most of my students last year ended up with UAIs at or above 70 (one was 63 and he was the lowest UAI but not the lowest ranked student in my class).
We did have students with UAI's below 30 but also half a dozen or more over 90. We only had about 10 Band 6s but over 50 Band 5s hence our low ranking as school rankings are done on the number of Band 6s in comparison to the number of exams sat.
Don't worry about last year's school rank as it has no influence on this year's result other than in indication of the type of clientele the school usually draws into its HSC cohort.