In writing your essay on "No One's a Mystery," I would first suggest that you pay attention to this very short story's title.
In studying the title, a reader can often find direction in discerning the author's intent for writing the story.
The fact that there is a diary is important in that it's a five-year diary; is the fact that it won't open symbolic? Is there foreshadowing with regard to the stubborn lock and also the narrator's use of the book in five years? Jack has a clear idea of what he thinks will happen, and what will or will not be in the book.
Our narrator is very observant. She notices the bleaching of Jack's jeans and the wedge of manure--does this detail speak to you?--in the heel of the boots he has worn in the two years he has known her. (We learn from this description that this older guy has been seeing the narrator since she was sixteen.) His car is a mess, and in trying to hide her from his wife passing in her car, he pushes her onto the floor where her jeans get dusty and she is face-up against a stinky ashtray.
If I were to write an essay, I might write about the narrator, and whether or not you feel she should look elsewhere or follow her dream of being with him. If so, I would speak to how he treats her, and what he really has to offer her. There is alcohol and sex, but she's really just a kid--even he admits it. Is there a future for her here? Does he believe there is a future for her here? Look to the last line that refers to the smell of mother's milk as being bittersweet. He must have children. How does he seem to feel about this based on his statement? (And remember that children are a part of her dream.) Does he seem enthusiastic about buying into the dream?
If you feel there is more evidence in the story to support another viewpoint, you could describe Jack, how he looks at life, and where he thinks his life is going? Does he see a bright future? Is he still alive with wonder and anticipation as the narrator is? How do you know?
Either topic would be acceptable because there is enough from the dialogue and the descriptions to support your point; supporting examples for each main point that supports your topic are absolutely necessary in order to get the best grade. If you prove your point of view with facts (details from the story), your "take" on the novel is as relevant and accurate as anyone's. Just don't skew what you read into something it's not in order to "make it fit." I tell my students that unless the story talks about outer space and creepy stuff happening, don't try to tell me there are aliens.
Also, state your point of view clearly in the introductory paragraph. Give a clear conclusion where you do more than restate your introduction (e.g., what does it all really mean?), and don't put new details in the conclusion. Good luck!