Who is the antagonist in It's Not the End of the World by Judy Blume ?

The sad part about Its Not the End of the World is that there isn’t a clear antagonist in the story, but many different characters could be seen as the antagonist. Karen Newman is a young girl whose entire world is collapsing—her parents are getting divorced, her teacher is not very friendly, and she doesn’t win the affections of her crush. Through the book, Karen slowly comes to terms with the fact that her parents are getting divorced and by the end, she realizes that it will be okay. However, there is a strong case that can be made for many different characters being antagonists in the novel—despite not having evil or bad intentions.
Mrs. Singer
Karen’s teacher seemed excellent before Karen had her. Back when she was Miss Pace, she was fun and popular as a teacher. However, the first day of class, after she had come back as the now married Mrs. Singer, she yells at Karen for not having her book open in class. Karen is not fond of her teacher and thinks that she yells too much.
Bill Newman
Bill is Karen’s father. Bill can be seen as a possible antagonist in the story because he is partially responsible for the divorce. Typically, it would be unfair to call someone an antagonist for simply getting divorced—marriage is hard, and things happen.
However, Bill can be seen as an antagonist because he doesn’t act like a rational adult nor a quality parent in the divorce. He is selfish and unreasonable in the story. For example, he leaves suddenly and doesn’t return without letting his children know where he has gone. He goes to Las Vegas to get a faster divorce, something that doesn’t serve the needs of his children, and he cannot get over himself enough to care about Jeff when he runs away.
Overall, Bill fails to be a good parent and person in the story. Nothing better demonstrates his selfishness than his refusal to carry Amy upstairs in chapter seven. Amy is the youngest and asks her daddy to carry her upstairs, Karen records that,

But Daddy said, “No, I’m not coming in.”

Ellie Newman
Ellie is Karen’s mother.
In the same way as her father, Karen’s mother is self-consumed in the book. She is erratic and flies off the handle at her children as a result of her own issues with Bill. She cannot look out for their needs effectively and cannot get over herself to look for Jeff when he runs away. One key instance of when Ellie is an antagonist in the story is when she tells Karen,

“You’re going to be a lot happier living in a house without constant fighting.” (Chapter 12)

The reason this quote makes her an antagonist is because I don’t think Ellie is considering Karen’s perspective. She will undoubtedly be happier without the fighting, but Karen is going to miss her father and their lives. Their family is falling apart, and her mother is relieved that the divorce is going well—something that she cannot get past to see the pain her daughter is feeling.
Aunt Ruth
Aunt Ruth seems like the most prominent antagonist in the story. She is Ellie’s sister, and she is angry at Bill. She openly discusses the idea of Ellie dating and marrying in front of Karen and is against Bill. The feelings she has are exclusionary of the children, treating them as if they cannot feel things as potently as adults. Aunt Ruth might see her actions as caring for her sister, but once again, she acts like the other adults and sacrifices the feelings of the kids for the sake of their parents. She tells Karen at one point,

“Karen . . . don’t be like that. This is even harder on your mother than it is on you. She’s very upset . . . if she sees you like this it’s going to make her feel even worse.” (Chapter 5)

Ruth’s point about Karen needing to be strong for her mother’s sake is entirely obtuse. She is so out of touch with the fact that Karen is a child who is not responsible for her mother’s well-being and is in fact in need of her own emotional reassurance and security that it makes her a villain in this book. I think if you are going to argue for an antagonist in the story, Aunt Ruth most closely fits the bill.
Karen
I would be remiss if I did not mention Karen. Karen is the protagonist in the story, and we are meant to relate to her through her narration. However, there could be an argument made that Karen is her own antagonist. She is going through a tough time with her parents. They are getting divorced, and she doesn’t know how to deal with it, but Karen does the worst thing for herself and keeps her feelings bottled up.
She can’t talk to her parents, as they are self-absorbed. Her aunt and uncle are not much help, she thinks her teacher hates her, and her brother is dealing with his issues. However, Karen could talk to her best friend, Debbie, about the situation, but she sabotages herself by keeping everything inside. I think it's weak to say Karen is an antagonist, but the argument could be made based on how she acts despite knowing and wanting to talk to people—she can’t bring herself to do it.