Writing affiliation as a high school student

First of all, I'm impressed by your ambition here. And I should warn you not to get discouraged if the math journal ends up rejecting your submission. Do you have guidance from someone experienced in academic publishing, to help you through the process?

As @Nate-Eldridge pointed out already, you should simply state your affiliation, and the country doesn't matter. But I will add:

• If the submission is not double-blind, there is a risk that people will take you less seriously when they see the affiliation. There's not much you can do about this, I think.
• High school students do contribute to academic publications, though I was surprised to see this. One was a coauthor of a paper in a very strong CS systems conference this year (others were at a university).

Good luck!

Simply list your high school as your affiliation, including the city and country where it's located (since there may be many high schools in the world with the same name).

It's irrelevant whether or not it's in the US.