I think interest in that type of pairing in anime has already peaked in years past, so I really can't agree that it is becoming a fairly popular trend. Maybe it
was a popular trend back then, but for these days I don't think so.
The examples you cited were all from not-so recent years, and I can't remember any fairly recent examples in anime that involve student-teacher relationships for the main characters.
Now big brother-little sister pairings, on the other hand...
