Math journal for high school students?

An excellent journal published by the University of New South Wales (my alma mater!) is Parabola, aimed at interested secondary students.

Mathematical Mayhem was a math journal intended for and run by high school and university students. It now runs as a section within the journal Crux Mathematicorum, published by the Canadian Math Society. It looks like the Math Mayhem section is accessible without a subscription. However, the focus seems to be on problem-solving, so maybe it isn't what you're looking for, but I thought I'd mention it just in case.

There is one dead journal Quantum