You're still in year 8! You still have plenty of time and opportunities to hopefully get accepted at a selective school. Being in a partially selective school is, without a doubt, better than being in a fully comprehensive school so you should be thankful for that! I must also point out that the school you go to has a minimal effect on your ATAR, meaning you must never lose hope of your ATAR. Your ATAR depends on your efforts and determination in your studies throughout year 12. If you aim to achieve a high ATAR, rest assured, you can do so at virtually any school.
What you should do starting now is take a deep breath and begin your preparations because you still have other attempts at selective schools and you must do your absolute best if you wish to get accepted eventually and achieve your personal goal/dream of going to your desired selective school.
All the best!