Got a high-school student doing publishable research and want to help them get published? Anyone can submit to the academic journals, but it is often difficult for a high-schooler to break into them, as the barriers are pretty high for anyone outside mainstream acadème. Luckily, there are a few journals that offer more hope for high schoolers.
For anything dealing with history, The Concord Review seems like a good choice. The claim that “The Concord Review is the only quarterly journal in the world to publish the academic research papers of secondary students.” I think that this claim is a little too strongly worded, as other journals have published such papers. Perhaps it is more correct to say that The Concord Review is the only journal devoted to the academic research papers of secondary students. I know one high school student who published there (actually, he was in 8th grade and taking community college classes, so he missed high school in both directions, but they published his paper). And no, it was not a relative of mine.
An AP bio teacher on the ap-bio mailing list suggested the Journal for Young Investigators, which is an online journal for undergraduate science researchers that is refereed by undergraduates. I know a high school student who has published there also with a project that he had done for science fair in his sophomore year (again, not a relative).
Another teacher suggested trying the Biotechnology Institute. They have a contest for student research called the International BioGENEius Challenge, and they publish a magazine for students. I suspect that they don’t have much student writing in the magazine, but I’ve not checked. There are a number of contests for high-school science students, some with good prizes (like ISEF), but this is not quite the same thing as academic publication.
For younger kids, there are even fewer venues, though Stone Soup Magazine provides a nice outlet for creative work (poems, stories, and art) by 8- to 13-year-olds. Muse magazine for ages 10 and up has their Muserology column written by Muse readers, but the style of writing is more chatty than academic.
If anyone has other journal publication options for high schoolers, add them to the comments.