I can't speak for Theoretical CS, but there is one Math and Physics journal that I know of that specifically publishes high school students: KöMaL https://www.

I can't speak for Theoretical CS, but there is one Math and Physics journal that I know of that specifically publishes high school students: KöMaL https://www.komal.hu/home.e.shtml https://www.komal.hu/home.e.shtml https://en.wikipedia.org/wiki/Középiskolai_Matematikai_és_Fizikai_Lapok https://en.wikipedia.org/wiki/Középiskolai_Matematikai_és_Fi... They publish in English and Hungarian. And, of course, there are no requirements regarding age or institution to publish on https://www.arxiv.org https://www.arxiv.org