Will a post on arXiv as a high school sophomore hurt my future academic life?
I suggest looking into publications that specifically target high school or undergraduate students. An example, but one that unfortunately no longer exists, is The Mathematics Student Journal (sample issue from 1971). A current one is Pi Mu Epsilon Journal. An organization that I've been involved with both as a student (mid 1970s) and as a teacher (late 1990s) is Mu Alpha Theta. These are USA based, but your teacher should know some appropriate for you.