Generally speaking, higher-ranked selective schools tend to set more challenging exams than lower-ranked comprehensive schools. As such, it is good to complete past exam papers from higher-ranked schools as part of your preparation across your subjects. Doing so will allow you to challenge yourself by attempting more difficult questions, and will also help you identify areas of improvement, which you can then gradually address as you revise content (and seek clarification where needed) and complete additional past papers.
As a starting point, you may wish to consider the top 10 schools, as it would be reasonable to assume that they (or most of them) set the more challenging exams. Of course, not all high-ranked schools are selective, but can still set challenging exams. Schools that you should consider when searching for practice material include:
- North Sydney Boys High School
- James Ruse Agricultural High School
- Baulkham Hills High School
- North Sydney Girls High School
With that being said, it is important to note that not all schools may offer all of the subjects that you are taking. For example, James Ruse Agricultural High School does not run Mathematics Standard, and you will therefore not find any past papers from this school for this subject.
I hope this helps!