>>715078806
>>715078927
>>715079005
>>715079204
>>715079429
>>715079821
>>715079976
Koyuki is, by far, the most vulnerable student. It would be not just easy, but absolutely trivial, to take advantage of her.
She's an idiot and she trusts you. Having Koyuki go along with what Sensei says would not be hard. Some harsh words, a bit of manhandling and you could get her to do anything. She wouldn't resist much.
Now, there are a lot of students that you could do this to. Haruka, Miyu, Misaki...but all of those girls have friends that would catch on to what was happening, and you'd have a difficult time after.
But Koyuki has nobody. Her "friends" don't trust a word that comes out of her mouth. They purposely keep her locked away for their own convenience. If she tried to tell them that Sensei did this and that to her, they'd tell Koyuki to shut up. They'd HATE her for trying to badmouth their precious Sensei.

Moreover, the fact Koyuki gets to have any freedom at all is because of Sensei. If she didn't keep cooperating, you could withdraw your support for her anytime, which means going back to the """self reflection room""" for who knows how long. She would quickly figure out that just letting it happen is what's easiest for everyone.