It followed all the rules. The adult teacher did not initiate physical contact with the student, etc., etc.. He waited until graduation and adulthood. But the story is lifeless and dull as fuck, even with potentially cute characters. That's because it's written according to rule. It might as well have had a good social mores stamp of approval, but it doesn't make it any more interesting to read. I mean, seriously! Did they have some kind of committee meeting to decide what was proper to include? It's like the story was written by a committee.
Sensei! Koi No Shitsumon Desu!