As far as I am concerned, most of this outrage never had a valid basis in the first place because literally the first panel of the story said what the context was: Senior High, as translated, which refers to the highest 3-4 years (depending on the school system) of secondary school before college. Assuming middle (ie the lower half of secondary school) or elementary school (primary school) is not reasonable when the first panel states it's senior high- all I see is a complete failure of basic reading comprehension followed by people losing their shit over what they imagined the details were but since this apparently happened on DeviantArt, such a failure doesn't really surprise me as much as it should.
The assumption reminds me of when Australia back around 2005 legislated that A-cup breasts promoted pedophilia and made them criminal to publish and possess adult imagery of even though the models are 18+ and thus quite legal to be involved in such material; they effectively declared an entire class of adult women to be juvenile regardless of their actual age because their boobs were too small and underdeveloped according to self proclaimed moral guardians. Because the argument otherwise is "the oni is too short and flat to be older than an elementary schooler, even though the very first panel of the story says this is a high school". All the counter-arguments I see here fall back on appearances and not the fact-setting exposition- said being said I guess it can't possibly be that the teacher is the really tall one in the scenario even though in the panels he's shown with other adults (as few as those are) he looks taller than them even when sitting and hunched over asleep.
Now the issue of a teacher/student relationship, even in a highschool, is definitely morally questionable to me, but it's also just a damn piece of fiction so I don't see the point in foaming at the mouth. It's an incredibly common story device that I don't see people freaking the hell out about in every other shoujo manga it appears in; and nobody raised the same moral complaints over the age gap about a 109 year old who is pretending to be a teenager getting into a relationship with a 17 year old either *insert "still a better love story than Twilight" jokes as needed* This point is a thing I am more annoyed by the general lack of rhetorical consistency in than I am by the actual substance of the content.