The story is quite simple. On the advice of her big brother, a girl named Mary brought a lamb to her school and caused chaos in the classroom.
This is how Wings interpreted the story;
Here are how some Blues Men (Buddy Guy, Jack Bruce, Buddy Miles) told the story about Mary and that rascal lamb;
Some gospel singers used the song as a spiritual with the added line;
"Mary had a little lamb and Jesus was his name."