I am currently a Junior and I am on the publication process of publishing a journal to a division in Elsevier (Impact Factor: 4.2). I am the single author of my paper and no professors were involved in the research. How significant is this compare to, let’s say, ISEF or other STEM extracurricular activities?
It depends on whether your paper gets accepted or not. I have hard time believing that a high school student not mentored by an experienced investigator can produce work that is publishable in a peer-reviewed journal. (And the wording of your post makes me even more skeptical.) Just my $0.02.
It was a process of multiple rejections from different journals and their feedback on my research, which I was able to improve the paper. I had to do it by myself because I have no connection with any universities and the research lasted 2 years (ISEF 2nd place).