If a magazine has rules for submissions, students should follow them. Submission guidelines are frequently printed in magazines. If a teacher does not have copies of the different magazines, he or she could write for the guidelines. If a magazine takes the time to create guidelines for the material they accept, students should take the time to follow them. Students should not submit many poems or stories in the same envelope.
Publishers seem to prefer a maximum of three items submitted at one time.
If a student has more quality items than that to submit, a child could wait a week or so, and then submit the extra material. The child could send it in a different envelope with another cover letter, etc. Another option, of course, is to submit the extra material to a different publisher. Sometimes magazines are more willing to accept material on unusual themes than on more mundane ones. A poem about rainbows, for example, has to be pretty wonderful in order to get accepted for publication in a magazine.