...is that he has sway over her grades. I know for sure, some teachers would never use grading to their advantage and all that...

I'm just one who doesn't believe it's wise to be having a relationship with a current student.

I have a similar tv example... an old UK Rompole of the Bailey where a young pretty teen girl conspired with her same-age boyfriend to get, sexually, what she wanted from her rather naive older teacher.

Rumpole was conflicted, realizing the teacher honestly got duped, but the law was the law.

Then again, that was old times. Today, Twitter has already decided who is guilty or not.