Well, for KS3, a story should have an engaging start to catch their attention. Develop characters they can root for. And have a clear message or lesson. Also, use simple and exciting language.
First, think of a theme or topic that KS3 students would relate to. Then, create characters with unique personalities. Plan a story arc that has some challenges and resolutions. Add details to make it vivid.
Just start with an idea that interests you. Then, build characters and a plot around it. Make sure it has a beginning, middle, and end.