Create a strange creature based on your imagination. A good way to get the children involved is to ask them to describe the creature and the class can vote on what the creature looks like, its color, what it sounds like, smells like, what it likes to do and other details that make the strange creature vivid.
Decide why the creature suddenly appears in the classroom. For example, does the creature want to learn, was it an accident, is the creature from another time, was the creature sent to the classroom as punishment? There are many reasons why a strange creature might appear in the classroom. The children will probably have many ideas on why the creature is in their classroom and not another classroom.
Give the creature something to do. Any character in a story needs to have a goal that he wants to achieve. What does this strange creature want? Does he sit in the classroom to learn, does he make jokes and interrupt the teacher, does he try to scare the children? Once the strange creature is in the classroom he should interact with the children in some way. This is the middle of the story.
Decide on a suitable ending for the story. What happens to the creature and to the children in the classroom? Does anything change or does everything return to normal? Does the creature become a permanent member of the class, stay for a week, take children with him to his home? The ending can be funny, sad, happy or magical. The ending depends on what idea you want the story to illustrate. What was the original purpose of having a strange creature enter the classroom? The end should be reflected in what happened in the beginning and the middle of the story.