Dark Stanley (formally known as Stanley Degroot) was once a cook at Springfield Elementary School.
According to Bart, the students always made fun of Stanley because he hadn't graduated from college (he was short two credits at MIT). One day, after years of such repeatedly teasing from the kids, Stanley finally went insane and became homicidal. Picking up a cleaver, he went after the students who mocked him, angrily decapitating them, and throwing their heads into a dish he calls "Kid's Heads Soup" as revenge against their mockery. However, despite having got his revenge, Dark Stanley was later be caught by the authorities, then hanged for mass murder, and buried in an unmarked grave, and when they came back the next day, the whole cemetery was missing.
After Bart told the story (as part of his prank), rumors of Dark Stanley began to spread around the school. These rumors include that Dark Stanley takes the skull of his victims and uses them as a toilet and urinates into it. He also takes the skin of his victims and makes them into footie-pajamas.
In Yokel Chords, it was first implied that Dark Stanley was just a ghost story that Bart used as a prank in order to eat the lunches of his classmates. Apparently when Dr. Swanson is seeing her psychiatrist (unable to get Bart from her mind), her psychiatrist suggests she is using Bart to replace her son who was murdered by Dark Stanley.