Isn't it because he's a rapist?

I suppose it isn’t the most ‘moral’ relationship in terms of just their positions and age, HOWEVER it should be noted that, it’s the student crushing on the teacher, Kawakami is an actual good person, there is consent in the relationship, and she literally questions if she should be romantic with a student which means that, unless you ask her, she never had any romantic thoughts of any student(or worst), all unlike Kamoshida.

Yeah you can question the relationship you can have with her and I can see how it is wrong , but you’re comparing someone whose dating a high schooler, to someone who probably encourage Sex Ed to have actual sex in it, with them being a volunteer. Both are doing bad things, but one is worst at least!