Teach the difference between mood and tone. The mood of a book overall may be somber or enchanting, but the tone is dictated by the way the author tells the story. You can write a somber story with a light tone. Stephen King is a master of such writing, especially in a number of his short stories. A perfect example of this is King's short piece "Autopsy Room Four." The piece as a very light, humorous tone, but it's a dark tale about a man who's alive while he's having an autopsy performed on him. The combination of the two elements creates a black humor that leaves readers shaking their heads.
Teach your students how to combine mood and tone to create unique short stories. Students should understand that creative writing is a medium that allows us to express ourselves in a variety of ways. Creating the proper mood in a story and putting a twist on that mood will draw readers and keep them there.
Teach atmosphere. Atmosphere encompasses everything surrounding a short story. The setting has its characteristic atmosphere. Interaction between characters creates an emotional atmosphere. Take students on a field trip to a cemetery at midnight. Have them jot down in their notebooks what they see, hear and feel. Have your students write a short story based upon that trip to the cemetery. Contrast this exercise with a trip to the park or a carnival.
Teach students how to use strong writing, relying on content rather than a string of adjectives. One strong, concrete adjective in the course of a paragraph is much better than a string of weak ones. Rather than write that an alley is dark and spooky, have your students tell us how the moon creates shadows that reach out and touch us.
Encourage your readers to read Stephen King, Edgar Allen Poe and other authors who are skilled at creating mood in their work. The best way to learn is by reading.