This is just my speculation, but I think the reason why Yabu and her group are always able to get away from trouble is because the Chairwoman covers for them. Like when Yabu killed the previous teacher’s pet parrot and didn’t get an expulsion. And I think the reason why the Chairwoman does it is because she wanted to create these “monsters” as a plot for her revenge and KNEW Jimi-sensei would take over for Class 1-A. But the big question is why? My guess is that she (I doubt tho) or someone she knew who was once his student, fell in love with Jimi (refer to first couple of pages of this chapter), but he turned them down because of his strong work ethic of being a teacher (which is why the Chairwoman says “I really hate ‘that side of you’”). Now, referring back to the rumor online about Jimi-sensei killing one of his student, maybe it’s related to the Chairwoman and her revenge. I believe he didn’t kill his student, but my guess is that maybe his rejection led to her suicide(?).