The title comes partially from Nevada admitting at the end of the story that she has never gone to school and does not know how to read. However, it also refers to the education that Barbara gets. She tries to set Nevada up by telling her Gilbert wanted to see her at midnight when the letter really said nothing of the sort. She clearly hopes that Nevada will be ruined, but instead Gilbert asks Nevada to marry him, and it's Barbara that gets schooled! As Nevada notes, "The joke's on Barbara, anyway!"
Thank you very much for your answer.