No, it's not even close to pedophilia. Shiraishi has graduated high school when they start dating and having sex which means he's 18 or will be within two months. He was around age 16 when Natori fell in love with him, which is also not "borderline pedophilia." It's actually the average age of consent in the developed world. It would have been very inappropriate for them to be in a non-platonic relationship at that point though due to them being student and teacher. I would also say that 16-year-olds and 25-year-olds probably shouldn't be dating or having sex even if they aren't student and teacher, the age gap creates too much of a power imbalance, 16-year-olds are much more vulnerable to coercion and abuse than someone in their mid-20s, but it's not pedophilia either.
This comment got in my head after I watched videos about Youtubers in their 20s who use their fame and influence to prey on underage girls, so I'm returning to add something.
Natori wasn't targeting and grooming his 16-year-old students. That's different and obviously would have been fucked up. I still don't think it would be pedophilia though, as I said it's legal for people of those ages to have sex in most countries (although I don't think they should), including in the developed world which overall has stronger child protection laws and cultures. It would be predatory and exploitative.
It didn't happen like that. Natori had a special relationship with a teacher which changed his life and saved his education so he wanted to have that type of personal connection with a student and make a difference like his sensei had done for him. All his previous partners had been older than himself, he clearly didn't have a fetish for teenage kids. They fell in love by accident. When he realized they had mutual feelings it did not make him happy and he wasn't planning on telling him, but then the poor kid *apologized to him for falling in love* which was heartbreaking. He then set the boundary that nothing could happen until Shiraishi graduates.
how old is the teacher? 25?