When I was reading the latest chap, I was quite irked mildly with the fluidity of the story and the "deception" of the teacher. But it was explained, the "before reaching 18 years old or die of starvation." So, sucking her first blood is an urgent issue.
It is like a story of a kind man who saved a scorpion from drowning. When the kind man was about to put the scorpion to the ground, the scorpion bit him.
The reflection is simply this: You can't stop the kind man to help the dangerous scorpion because it is his nature to be kind. In the same way, the scorpion can't stop biting the kind man because that is its nature to bite.
Thus, in analogy, you can't totally blame the teacher of deceiving both for the sake of saving Iris' life. Yes, it is unfair for Sara but that is "survival of the fittest", kill or be killed. And it was even through Sara's eyes that we can see the sadness in the teacher's eyes forcibly hiding through acquiesced flirting and joke.