Before visiting any online sources for a story, it is highly recommended that you view what your classroom has available (the sections with the books should be highlighted).
Did you go and search? I don't believe you looked hard enough. Make sure to check short story binders, the Year's Best Collection, as well as literary magazines, such as The New Yorker.
You seriously think you have checked every resource in the room and there is absolutely... positively... nothing? Check one more time.
If you can find nothing there, here are some other options:
A collection of public domain stories that are available for reading.
A collection of short stories by authors contemporary and classic. Stories are organized by genre.
A collection of public domain stories. Click on the read option to see the stories they have available (you will need to know the story you are looking for beforehand).
The Guardian Collected Stories
The Guardian published a nice series of short short stories that I've pulled good stuff from before.
Short horror stories. Found (and loved) by students.
Short Stories
