A S Byatt 
Raw Material (Storycuts) 

A downtrodden creative writing teacher despairs at the efforts of his students, at turns nakedly autobiographical or fanciful beyond reason. When an elderly lady begins submitting work in accordance with his 'write what you know’ directive, his enthusiasm for the craft re-emerges. However some sources are best left unexplored. Part of the Storycuts series, this short story was originally published in the collection Little Black Book of Stories.