High school writers looking for a place to publish their work should have a look at The Blue Pencil Online, a project of the Walnut Hill School for the Arts, in Natick, Massachusetts (USA). Their standards appear to be quite high, so for a young writer of real talent and ambition, the Blue Pencil just might provide the right sort of challenge.
For a brief article about the site and the school, see this page from the National Association of Independent Schools.