This is a good exercise to try applying to your own writing; you don't need a classful of students to do it for you. Once you've finished the rough draft of a story, take a moment before you revise and ask yourself why you wrote this story. Was there an idea or a truth you had to tell? A character you wanted to describe? A social tension you wanted to explore?
If you find it difficult to identify the driving purpose of why you wrote the story, it might be an indication that readers won't find purpose in it either. Not every story has a big, overarching statement to it ("This story means you shouldn't tell lies!" "This story means freedom is good!"), but most do have something urgent and imperative that has to be told or implied. If there is no imperative, then why are we reading? Why are you writing?
The point is not to question yourself until you're a trembling wreck. The point, rather, is to begin looking at your work with a more critical and discerning eye, to figure out what will be lasting about your work -- what will make it stay in a reader's mind. What can a reader learn from your story? What moment will make him think, "I feel the same way"? Make sure your story has an imperative that answers this question, in one way or another.