Very simplified ur atar is a collation of 50% internal school marks and 50% ur performance in the HSCs. Ur internal school mark is decided by ur rank for a subject, say 12/70 for Biology, and then you recieve the mark of the person with the 12th top score in the HSCs. So ur hsc is 50% ur hsc mark and 50% the 12th person's hsc mark.
The better your school, the better you'll do and there's also scaling involved. For example the 50/70 for James Ruse is in a very competitive environment and will have their internal school mark with a multiplier to do better than the 50/70 for a 100s school. Lots of more complex stuff but I think thats the basics