Well, I think it’s…
Well, I think it’s obvious that there is some problem to be considered here, but I’m a bit puzzled that you treat “teaching and evaluating” all of a piece. I took a class in statistical methods that my father taught several years ago; the arrangement that we worked out was to ask the Department Chair, as a favor (since it concerned a single student only, and the course was in his area of specialty, it wasn’t that big of one to ask) to grade the exams that I took. It may not be possible to arrange something of the sort in every situation; but if it’s not possible to arrange a third-party auditor for the kid’s grades, well, that’s as good a reason as any to have the kid enroll in somebody else’s section.
It seems likely to me that there are some parts of the student-teacher relationship that are worth taking a harder line on than others.
Anticopyright.