this stories problem in my opinion is that its characters lack believable motives due to a lack of elaboration making their actions make no sense and causes the narrative to become less than convincing as a plausible conflict.
What motives do you want? It's teenage drama. Teens don't need elaborate motives to do stupid shit. That's what teems do. I don't know about you but I can totally related to that.
That been said, it's not like the characters have no motives at all. We know that Yuni's cheating on Nanase because she feels neglected, for instance. People cheat for way less than that, you know? We also know that Fukko is messing with Yuni and Nanase because she's kinda traumatized over her unrequited love for her childhood friend. So there are motives aplenty.
Anyway, my point stands. There's no basis to claim this thing is badly written. It's consistent and the pacing is good and while things are exaggerated for the sake of drama, it's not really implausible.
this isnt about what i want from the characters,its about what the characters need in order for their actions to make sense beyond their so called age which truly doesnt matter at all.these characters would act the same way even if they were older or younger,as the age of a character hardly ever actually make them act their age.
in regards to bad writing:
yuni is "neglected" by her girlfriend so she makes the leap of logic to start cheating as a solution to her relationship problems,and yet some readers claim its because she is a teenager.
fukko was "betrayed" by her unrequited love so she makes the leap of logic to traumatize another person way worse than she was,and yet some readers find this hot so its all good.
nanase is in "love" with her girlfriend so she makes the leap of logic to prioritize her club activities over her beloved girlfriend,and yet some readers claim past trauma is at fault.
this manga simply hasnt truly elaborated enough on the reasoning behind its characters actions forcing the readers to speculate in order to make sense of this mess of a story,and there not being a deeper reason for the characters actions other then the surface reasons causes the story to be very shallow.