There are many avenues for getting student work published in professional magazines, one of which is Stone Soup, a "literary print magazine written and illustrated by young writers and artists." They are a nice publication that offers a platform from young talent like those in your own classroom.
You can encourage your young creators to submit work to Stone Soup by following their guidelines.
Students must be 13 or younger, and submit their work by mail. Also, visit their archives for previous examples of printed material. Consider investing in a subscription to Stone Soup. Their stories would be good reading material for classrooms, especially for silent reading time. Students are even paid if they're chosen for publication!
What better way to encourage students to hone their crafts and take writing and illustrating seriously? By introducing them to Stone Soup, you may very well be grooming a future literary and artistic career.