There are a lot of weird and bad relationship dynamics that anime (and to some extent, Japan) seems to give a pass to. Like in Cardcaptor Sakura, Sakura's mother wed the student teacher 9 years older than her when she was 1 and he was 25. Then one of Sakura's elementary school classmates receives a promise ring from their teacher and everyone acts like it's so cute and fluffy as opposed to inappropriate and predatory. That's a cute fluffy anime and it's still problematic as hell. And let's not even start on the incest.

Anyway, now that you mention it, it's funny that we didn't get any student/sensei matches from that whole mess that is Naruto.