In terms of applying and receiving offers from selective schools, you must ensure your academic performance at your current school is of high standards across most of your subjects. Of course, if you are currently performing/can perform well across all of your subjects, that will definitely put you in a more favourable position in terms of your application. You should also consider any external academic achievements you've had, including mathematics competitions such as the Australian Mathematics Competition (AMC) and International Competitions and Assessments for Schools (ICAS). Additionally, you should consider times you've participated in extracurricular activities. If you haven't done any of those, you should definitely consider participating in both as including evidence of your participation and good performance (if applicable) in such events/activities could help you become a more competitive applicant.
You must also ensure you perform to the best of your abilities in the school's entry test (where applicable). For example, Penrith Selective High School requires applicants to sit the Higher Ability Selection Test (HAST). Your entry test results are also important as they are considered by the school you're applying to. Good performance in this case can be achieved through effective and thorough preparations prior to the test.
Lastly, since you may also likely be required to attend an interview (which is conducted as part of the selection process) you should make sure you are prepared for the interview. Depending on the academic year in which you are applying to a selective school, you could expect similar questions to these:
- What can you bring to the school?
- What extracurricular activities are you involved in?
- Favourite subject and why?
- Why do you want to come to this school specifically?
- What do you want to do at uni?
- Subjects for year 11?
- If you are applying for an agricultural school - Why choose an agricultural school?
I hope this helps!