I know it's not PC to say this, but it is VERY likely, in my opinion, that this relationship did start out as one where he was truly trying to help her, and he got caught up in it and lost his brain completely. This isn't like some monster who snatches a small child off the street and then rapes her violently and strangles her and leaves her posed and naked to be found by the community.
This is a man who is a teacher, who had an inappropriately close relationship with a student who was damaged by a childhood filled with abuse, and he lost all his sense and did the unthinkable, and unrecoverable - he encouraged her to leave with him.
Life happens in shadings of greys, not in blacks and whites, IMHO.
And I have eyes in my head. I went to high school in the late 70's, and I did see cases where male teachers were having affairs with students, and in one case a female teacher having an affair with a male student.
And I think those who aren't able to see that maybe just don't know what to look for. It's there. It's there wherever groups of humans exist, they find each other and in their brokenness, locate the ones similarly broken.