Lots of kids and teens I know love to write, and are interested in getting published. They might not want to write a novel and be represented by an agent yet, but they'd love to be published in a magazine or have their stories online. Below are some links to sites where kids and teens can put their stories online or places where they can submit their stories to magazines. If you think of any more sites please let me know. Note that I do not control any of these sites and cannot be responsible for content.
This is a site specifically geared towards girls. The company has a website where, if a girl is a member, she can post stories and poems, along with art, on the site to have other girls critique the work. The projects can be submitted for consideration in New Moon magazine. Recommended for girls 8-12.
This is a magazine where kids can submit their stories, poems, and book reviews. Editors pick the best of the bunch and the stories are published in the magazine. Submission guidelines are on the website, and authors recieve payment for their stories and poems. 8-13; you can submit stories and poems until you are 14 (14 year olds and up cannot submit stories though they can read the magazine).
This is the nation's largest magazine "for kids, by kids". You can submit cartoons, stories, poems, photographs, artwork, games, editorials, and poetry, and whatever else you feel like to the magazine. They're looking for the best work for students ages 8-16.
The magazine editor says that if readers have a story they'd like to tell, she's interested! Only guidelines are that the stories need to be your own work and be less than 200 words.
This site is only for poetry. It accepts original poems from children. The poems can be submitted online or by mail.
Kids can also be published in certain school-distributed publications like Time for Kids (http://www.timeforkids.com/) and Scholastic News (http://www.scholasticnews.com/).
This is a site for students in grades 6-12 and is supported by teachers. They have a New Library of of Young Adult Writing, which "showcases outstanding works of fiction and nonfiction by students in grades 6-12". You can enter writing contests and teachers can submit their students' best work.
This is a site for those 13+ run by HarperCollins. Writers can submit their own work and critique others, discuss writing on the forums, and interact with other writers. Every month the top stories are critiqued by HarperCollins editors for a chance at publication.
The Blue Pencil
This is a site for high schoolers run by the students in the Writing & Publishing Program at Walnut Hill School for the Arts. They want to publish the best literary work by young writers around the world. It also offers a scholarship prize.
The Claremont Review
An international literary magazine for young adults.
A monthly print magazine, site, and book series written by teens for teens. You can submit your stories and poems to the site for a chance to be published in the Teen Ink magazine.
An Australian website designed to help young adults with story writing. The site is intended to be a "writing companion and a resource to draw on as you craft a narrative".
Hopefully these sites will be helpful.