Too bad there isn't all that much of that. We have opinions that are neither explained or even well thought out. And we have people -- including you -- using this story as an excuse to get on a soap box about their personal beliefs regarding ages of consent and will-somebody-think-of-the-children and hiding it under the fig leaf of criticism. Probably because they don't get enough visits if they post their diatribes on their tumblr blogs instead.
In this doujin, we have an adult authority figure using her position to get alone with one of her underage students, doesn't respect the student's physical boundaries, uses cheap tricks (such as being physically intimate with another of her students) to confuse and upset the student, and of course makes no bones about pushing emotional intimacy on someone who's immature, confused, and under her authority.
It is by no means impossible to write a romantic teacher-student story, I'm pretty sure there are a few on this site. But this is not one of them, because the teacher acts in a way indistinguishable from a predator. At best, she's an irresponsible spaz who leaves a trail of broken hearts among the teenagers entrusted to her care. That would be bad enough, and I find that a charitable interpretation.
Something really important is to keep the critcism grounded on the goal the artist had when doing his or her work, in this case you're projecting your values onto the work because the artist only did whatever he/she found appealing/would sell well.
With this in mind a more adequate critique would be that the story is shallow, unidimensional characters and all around lazy.
