Life Is It's Own Book Waiting to Be Written

The school I went to, you get grades to writing stories and bringing them to life.To think of whatever you can imagine and put it on paper and have the whole class clap and give their views on how great and thought provoking it was. And I should know what that feels like since I was one of the best writers in that class. (So my English teacher claims.) Yet I know most of you are sensing a 'but' somewhere.
My problem is a story must be 300- 500 words and done in 1 and a half hour. With my style of writing its impossible. I usually run out of time or my plot is not properly explained. My well thought out stories go to waste and get the falling grade.
But even though Im not gonna get any A's here to raise my G.P.A I fell like my stories should go somewhere. So here's some of the stories I've written and don't be afraid to send me some prompts. you can massage me for the rules.