Yumi mentioned multiple times that Sensei before game started gave her detentions after class and many of them involves sexual harassement. So in a way Yumi expected this would come and she couldn't do anything about it since she's the unwelcomed student. And afterwards Sensei kept apologzing to her and helped her in ways other never did. Remember that Yumi is a highly insecure girl.
I can't say more without great spoilers. But just keep in mind that composition, it will show up again in exact same manner in a Happy Event much later.
Or, if you really really want the truth, I can show you.