When teaching writing, one strategy is to have students model their work after a master of the genre. For the scary story, Edgar Allan Poe is a well known American author. As a matter of fact, his work is now in the public domain. That means people can copy his work from sites such Project Gutenberg for free.
Read Poe's Work
For middle school students, two great stories to read in class are Tell Tell Heart and The Fall of the House of Usher. He, of course, has many more short stories from which to choose, but these are great for the middle school or even high school classroom. The strangeness of the tales coupled with the gothic devices that Poe uses will give students ideas for their own stories.
Tell-Tale Heart
The "Tell-Tale Heart" is written from the point of view of the murderer. The reader does not know his or her name and is abruptly drawn into the strange story. It is not full of gory details, as in many of today's scary stories, but relies on the narrator's madness and the "eye" of the murder victim. Madness is a theme in both of these stories. The narrator does end up killing the old man after many attempts and stuffing him under the floor, but the details of the bloody deed are not the focus of the story. It is the narrator's fixation with the eye and then the beating heart.
Fall of the House of Usher
In "The Fall of the House of Usher," madness is a theme too. The narrator is asked to come to his ailing boyhood friend's home. Poe uses the typical Gothic genre devices--gloom, decrepit home, dark pond, tombs, etc. His friend has a twin sister who appears like a ghost at one point, is buried alive and then falls dead on her brother when she breaks free from the tomb. The house then literally falls apart.