Where was Nagaoka during the Pizza-ta Incident? In fact, where was he during most of Kitahara's in-class bullying? Given what a nice guy he is, you'd think he would stand up for Kitahara once he saw how she was being harrassed.
There's much you can do for someone who shuts himself in, even by yourself. Trying to act alone against (more or less) organised bullying on a target who had given up, on the other hand, is a social suicide. And in Japan it's much more difficult for one of different gender to help each other (you can thank their social awkwardness for that). Kurosawa was a loner because he didn't see the need to form relationships, and in this case Nagaoka's tries would eventually succeed (as evident later when Kurosawa really acknowledges him as a friend). Kitahara on the other hand was alone because she was a very bitter person and didn't care about any of them, and to top it off there was the bullying. When the bullying on Kurosawa had started, he wanted to be on good terms with Nagaoka again, which is what allowed him to reach to Kurosawa.
At the very end of the series, Kitahara has cut her hair short and is seen going back to school again. She looks like she's much more willing to interact with people and is thinking positively, compared to how she once was. But in the novel epilogue, she acts much like her usual cynical self again. What happened to all her character development?
Trying to appear as a different person while still being herself when with old acquaintances, perhaps? She has a mental problem that won't just disappear without help that easily, even if she did appear to be happy. Besides, she doesn't appear all that bad - blunt and coarse, sure, but that's not bad compared to how she were before.
Why couldn't Kurosawa just fap at home?
With someone like Kurosawa, who lived to violate and disrespect the women he fantasized about, it wouldn't be as fun to just do it at home. Doing it in a place where he's much more likely to get caught probably not only makes it more exciting, but also probably makes him feel like he's further humiliating them, as if to say "Not only am I fapping to you guys, I'm also doing it in a place you'd never think I would." Later in the series, though, it's probably just a habit that's hard to kick