If you are in a selective or private school where the academic competition is extremely strong and you might not be in that top 15-5% of your year level, but you have strong ACT/SAT/AP scores, how will that be viewed?
There are a number of relevant factors:
Where are other students with stronger class rank than you applying? Schools like Harvard or Stanford rarely take 2 students from the same international high school in Australia and, all else held constant, will take a student who is the Dux or a top scholar over a lower ranked student.
How strong are the ACT/SAT/Subject Tests/APs of the students ranked higher than you? Are you taking more than them? Are you doing extra academic competitions to differentiate yourself?
Does your school publish specific class ranks that enable you to be directly compared?
Will the higher class rank of students above you translate to stronger academic references than you?
In general, the SAT/ACT are only regarded as screening tests. You need to beat the ‘relevant’ average for you (see my other answer to your question) so the universities won’t place very much emphasis on an extra 50 points on your SAT over somebody else.
The Dux (top student of a school) with a 1500 SAT will beat the 2nd or 3rd ranked student of the school with a 1600 SAT score from an international school in most cases. This is because Dux or your GPA is a more intensive measure of your performance at your high school over several years. HOWEVER, the student with the lower ranker can beat the Dux if they have exceptional EC, LCS, higher or the same SAT/ACTs and better essays (within reason).
What you need to do is: get your class rank as high as possible, don’t be complacent and think your SAT score will carry you through - it won’t. Go hard with all areas of your application and don’t abandon the hardest competition - namely that in the competition because that will expose a big vulnerability in your candidacy if your percentile rank falls too much.
Good luck! Feel free to follow up.
Yes - just like Jamie said, you really want to find a way to distinguish yourself, and doing that in some aspect of achievement other than standardized test scores will likely yield a better outcome for your situation. What is special about you beyond your grades and scores? What do you feel passionately about in your activities that you can perhaps take to the next level? Do you contribute to your community in a meaningful way? Are you an awesome artist or musician in your free time?
Obviously continue your efforts to get your best possible grades. However, some self-reflection about your own uniqueness can perhaps help you find the best way to set yourself apart from your peers.