Now that I've had some time to think about this, even if we take into consideration the silliness of the teacher storyline, it could also be argued that she doesn't want her bottom lips plastered all over the place regardless. So the blackmail storyline still works.
For those who argue that she takes topless pics, there's a huge difference between topless pics and a video showing you getting fucked.
It's the difference between posing for playboy which is considered somewhat classy and posing for hustler.