Chika's more or less called Yumi a liar when it came to the creepy detentions in the past:
I don't really see any conversation about Sensei being in the wrong, going well, for Yumi, when it comes to Chika. Even Ami
and Ayane seem to find Chika's feelings for Sensei concerning.
Keep in mind that Chika was likely staying in a room close by, the whole time. Yumi doesn't really have an excuse for not saying anything to Chika about the secret slumber party with Chika's boyfriend without Chika. Let alone an excuse for the hint that Yumi and Sensei had fooled around prior, based off their conversation that Chika overheard.
Overall: Unless Yumi turns into a master bullshitter, she's probably in trouble, lmao.
I do think Chika wants it all to be a delusion, though, so at least that might help Yumi's case.