It seems to me that the author is interested in writing a story where things are not as dark as they seem. People can fall to bad impulses, but also learn to be better when someone gives them the chance. A lot of sympathetic qualities were written into Naoi, Kurumi, and also Kokoro. Some of them seem like for-the-evuls characters like Isanuma and Kudou, but we also know the least about their backstories. However, we've never seen them question their own behavior yet, which Naoi, Kurumi, Kokoro, Akane, and Isanuma's friend (I forget her name) do. That doesn't mean none of the latter group will fall to the dark side, but I can't write any of them off.
The adults meanwhile are a lost cause. They've already decided who they were going to be, are deep set in their habits, and lost their chance to improve. Their fault for being over 20 in a manga. I'd like to be wrong, but I think the author wants to give the kids the focus. The adults are only a plot device as well as a way to show what can happen if you follow certain impulses to their end, like Naoi and her father or Kokoro and Kurumi's mother. I don't know if Kurumi has an equivalent; maybe the teacher who tries to please everyone and keep up his image, but without Kurumi's sense of ethics.