In real life, they are abusive. Teachers who engage in sexual relations with their students, even if they are not minors, are exploiting their position of power. If the students are minors, that only exacerbates the problem.
When I taught university, I would never have considered having a relationship with one of my students, even graduate students, despite them all being adults. I had a colleague who began a relationship with, and eventually married, a grad student, but only after she had completed her course work. She was also in a different subfield from him, and he was not involved in her dissertation research.
I was surprised and taken aback when I watched the first episode of
Hanamaru Kindergarten where the mother of one of the kids was the protagonist's high-school senpai. In a flashback we discover that she had become pregnant by a teacher, then married him and left school before graduating. I thought the whole story was treated much too nonchalantly.
About a decade ago we had the egregious example of
Pamela Smart who seduced a fifteen-year-old student then convinced him to kill her husband. Smart used the threat of cutting off their illicit relationship to blackmail the boy into the murder. Smart is now serving a life sentence for conspiracy to commit murder and several other charges. A novel based loosely on these events,
To Die For, was adapted into an excellent
film by Buck Henry, though Nicole Kidman's character was not portrayed as a teacher but as an ambitious local television personality.