If I understand correctly, Nene expressly decided to go to the same high school as Amane, even after she had decided to break up with her.
If she really believed that cutting ties was necessary (either for her own psychological well-being or to protect Amane from the consequences of the toxic way their relationship was making Nene feel), she could have gone elsewhere.
So going to the same school and then torturing Amane emotionally negates any positive spin you could put on her decision to break up.
I’m saying that I do dislike the writing choice here, because to make it work (i.e., to make Nene seem more like a conflicted adolescent and less like a sadistic narcissist) we would have needed more and earlier insight into Nene’s decision and more nuance in how she treated Amane (and Kou, for that matter—that business of “flaunt the new relationship in the place that used to be the Special Place with the old girlfriend and where the OG now works” is almost as shitty a thing to do to the new girlfriend as to the old).