This is exactly how this shit actually goes down at school lmao. "Oh we're going out now," is how basically every teen "romance" I've seen and experienced starts. Maybe it's different in Japan, but this feels very authentic to me.
She's acting like a yandere/jirai kei type but since the story don't uses the usual tropes and visuals associated with them makes it more clear what the message than It would be with those shorthands in a way; Like it's easier to see the mistakes she's taking and why and since she's acting like she always was i can see that everyone can commit that mistake or at least how are we oblivious to the shit we make.