F. Scott Fitzgerald, "Bernice Bobs Her Hair" (1920) from F. Scott Fitzgerald: Novels & Stories 1920–1922:
I have a vague recollection of reading this in school once, but I have no recollection of my reaction of what we talked abou then. If you've never read it, the basic outline is (a) popular cousin Marjorie educates her visiting and unpopular cousin Bernice; but (b) when Bernice starts to become more popular than Marjorie, even stealing the attention of her best beau Warren, Bernice issues an ultimatum: you've been talking about bobbing your hair, so if it wasn't a bluff, let's go do it; and (c) being caught in a damned-if-you-do, damned-if-you-don't situation, Bernice bobs her hair and feels awful about it now that she's no longer pretty; so (d) she leaves early, making sure to cut off Marjorie's hair on the way out of town.
The POV is detached, a little ironic, and omniscient, giving us views from cool Marjorie's, lovelorn Warren's, and sentimental Bernice's positions. So in the discussion between Marjorie and Bernice on whether or not Little Women-style stories should be their models, we get little views from each mind. But let's cut out that ironic POV tone for a moment, and note that the plot is almost textbook high-school makeover rom-com: with a pretty girl helping a less popular girl to be popular and then the helpee turning into a threat that must be destroyed, the only thing we're missing is where some boy comes out and tells Bernice that he always kind of liked her before and/or now he realizes what a monster Marjorie is.
Plot- and character-wise, what sets this story apart is the bitter, bitter end, where Bernice loses the boy, runs away from the town early, and inflicts vengeance on her cousin. It's a great example of how certain formula can be twisted by upending some expectation. (Not that Fitzgerald is exactly dealing with expectations or formulae here; only that from our reading position, the power of this story is what sets it apart from the stories we see now.)
Bonus: As the LoA page notes, a lot of this story comes from Fitzgerald's own advice in a long letter to his sister on what boys like.