Even when Sensei mentions Yumi could rip off his dick, her hands are actually free, and if she's actually wanting to do that, then I'd think scratching him or gouging out an eye, kneeing him, etc, wouldn't be much of a difference. (Edit: or, you know, just punching him.)
I suppose another way to put it is that her heart wasn't in it from the start. Or at least, that seems to be what's implied.
Not like Nodoka would have known about any of this, or much about Yumi, beyond her bullying Futaba, in the first place.