Let me tell you this, if you're in the top tier of the grade or bottom tier, others performances will not effect you significantly, if at all.
If you're first in your grade and a person in your grade achieves a mark of 60(say, the lowest mark in the state), this will have no effect on you whatsoever.
If you're last in your grade, and the top person got 100, they will NOT "drag" you up.
The only "dragging up" you'll see is an illusion, seeing as if you are average in a top ranked school, a student of your calibre would be seen as the top tier in a low-ranking school, so in the end it'd be the same.
The only real determination of your HSC/UAI is YOU and YOU alone, not your school or your subjects.
Because they either give average rankings in a low ranking school, thus it is highly unlikely they'd be able to get a high UAI.
If a person from James Ruse gave us last rankings for all his/her subjects, you cannot assume they'd get 90+ regardless, they'd have to do well in the HSC to obtain a 90+.
PS. I've heard stories of 70-80 UAI's at James Ruse.