Why would Chika say that? As I can see, there are 2 possibilities: either Chika is mega delusional or detentions didn't exist in her mind at all. If it is the first, then we know absolutely nothing new. But if it is second, then it means either Chika's memories got altered somehow or detention never happened... in this timeline.
Expanding on your second possibility a little further:
It highly depends on what exactly Yumi told Chika, word to word. Yumi's tendency of exaggeration and uses of exclusive qualifiers in her sentences would make her less believable regardless of what truth was inside Yumi's head and regardless of whether Yumi was telling that truth or not; she simply is an unreliable information source.
From what I am seeing, Chika would likely believe "Yumi has received detentions from Sensei before" no matter what because Chika knows Yumi's overall demeanor. Chika could see it happening with or without prior confirmation from either source, but what Chika doesn't buy is only the creepy part because of the Sensei's image she established in her own head.
So the more possible option here is that rapey detentions didn't happen in this timeline but Yumi remembered them. When informed, Chika was being "yeah yeah sure sure" with Yumi about the subject, thus limiting the misaligned memories to Yumi.
What I have some doubts about the detentions is that the newest conversation between Sensei and Yumi regarding detentions quickly leads to "this must've been a past iteration thing" without considering the possibility of "this might've been a pre-cycle Sensei thing". Personally I believe the former because that's the more interesting option and if rapey detentions actually happened then Yumi's behaviors toward Sensei would be quite self-contradicting, but nothing they said really disproves the latter and I feel like a QED effort is missing here.
