How can I increase my chances of getting into a good boarding school?

I got 86% on the SSAT which is not half bad but definitely not the best.
I really want to go to a boarding school in the US, and I would like to go to a good one if possible, because I wouldn’t want to spend so much money studying abroad for just average education.

I speak fluent English, and I think my interviews went pretty good. I recently had an interview with a very good school that I would love to go, and it actually went amazing.

However, I have zero extra-curriculum activities. I don’t play sports or play instruments

I won third place at a national essay writing contest when I was younger, and I won second place for an English writing contest in my school, but that’s not really a huge thing.
Oh, and I won third place for a national English exam.

I have straight A’s except for one B for learning Chinese letters.

I’m really excellent at math, but I have no way to show it. I have nothing to show my leadership, since I wasn’t class president or the student council.

Another downfall is that I need a lot of financial aid as an international student, bc my parents are divorced.

How can I increase my chances at a good private boarding school? I’m not aiming for Exeter or Andover but maybe for Hill or NMH.

In Korea, middle schools don’t have debate teams or a variety of sports teams, and we value STEM so much more then sports and arts,
so there aren’t many opportunities for extra-curricular

Is there anything I can do right away to add to my application?

If there are students who got into a good school from Korea, pls tell me thank you