Questions/thoughts to ponder when writing a short story:

What do you want to write about?  F
What does the main character (the protagonist) want in the story?  What is their motivation? Are they after something very specific, such as a date with the cutest boy/girl in town, or is the story about something more broad, such as a person trying to survive through their first year away from home or dealing with the experience of how things have changed in their hometown since they’ve left?  Perhaps your main character has a friend who gets in a car wreck, or tries to rob a bank, or perhaps they are the star of their high school football team and just got injured or benched.  What kind of struggle/conflict do you wish to write about?  Life is full of them.  Any sort of struggle is material for a story.  Think of what struggles interest you.
What is the main conflict of your story?  .
How does the protagonist change as a result of trying to deal with this conflict?  What has happened to the protagonist from the beginning to the end—have they been affected?  Is it life-changing? Have they learned something about themselves/the world at-large?
What is the setting of your story?  It doesn’t have to be a story set in either:  a college campus or your home town/high school.  Though, these options would be the easiest, since you know a lot about them.  You know how to write about these places and make the story/problems real (write what you know, as they say). But what else do you know about?  Surely there are many, many other things you know about and could write about.  If you don’t want to write a realistic story—if you want to write genre fiction (murder, suspense, horror, romance, science fiction, sports—stuff for television shows), you certainly can.  If you do, however, I challenge you to try and make the characters real and make their problems real.
Anything can be made interesting, with honest insight, thought and emotion, but on the flipside, anything can also be made boring, with flat characters and a predictable plot.  Use your imagination and have fun.  I personally enjoy stories where unexpected things happen and people have surprising thoughts, say surprising things, or have interesting emotions/reactions.  I especially like it when something is unexpected but completely believable.  It doesn’t have to be an event; it can be a character’s reaction to something or someone which somehow causes a change in that person or reveals something about them that they (or the readers) didn’t know.
Stories, in the end, should hopefully show us (show through a character’s actions, not just  tell) something about the main character/the world/human nature which we did not know or could have guessed about them on the first page.  Beyond that, it can be about absolutely anything.  A character, a conflict, an ending.  If you have these three things, you have a story.
So, what do you want to write about?