Chapter ten of 'The Grammar of Fantasy' by Gianni Rodari talks about Dadaist and Surrealist games to look for a 'fantastic theme' that will help your class to write stories.
An easy one to set up in the classroom is the Casual Sintax.
The students, one after another, have to secretly answer these 6 questions:
question 2: WHERE WAS HE/SHE/IT?
question 3: WHAT WAS HE/SHE/IT DOING?
question 4: WHAT DID HE/SHE/IT SAY?
question 5: WHAT DID PEOPLE SAY ABOUT HIM/HER/IT?
question 6: HOW DID THE STORY END?
The first pupil answers the question and folds the paper like an accordion. He passes the folded paper to his classmate, who will answer the second question, fold the paper again and pass it to the third one and so on until the paper accordion will reach the sixth and last student of the chain. Once he has answered the last question you can unfold the paper and read the answers following the order of the questions. It'll be hilarious and perhaps you'll find material to write a whole story.