If Yumi ever develops an attachment to MC and finds out about Futaba, I would expect violence.
If/when Yumi becomes intimate with MC and Chika finds out, there will be violence (possibly against MC)
Self-harm is violence, and Rin is Rin.
Noriko is predictably violent. But fully half the girls could easily reach that point given the right set of variables, especially in the lead up to a reset when everyone's "settings" are wonky.
I don't think Yumi will actually care much about Futaba. The hate against her was never true hate, it was rather disappointment that she would not stand up for herself and fight (something Yumi has been doing her whole life, and that is the measures she is using here on Futaba as well). I don't even think she dislikes her at all, imo it is mostly projecting her own fears into Futaba and that makes her angry. In a very weird, Yumi-ish way she cares for her that way (which ofc is tots fucked up and I am not trying to justify it, merely trying to explain it).
I also don't think Chika will be any violent would she ever find out about Sensei and Yumi. Rather she would be happy that finally Yumi found someone she opened up to, sth she didn't even really do for Chika. Ofc, Chika would be incredibly hurt, but she is the kind of person who would accept the pain in favor of Yumis happiness.
However, I fully expect violence if Yumi finds out that Sensei bangs everyone else aside of Chika and Chika finds out that as well and gets hurt by that. THAT is where Yumi would draw the line and that is where she will become violent, because both are family (not by blood but in spirit). Also Yumi would massacre Sensei would he ever hurt Chinamy, but she is the only one safe from Sensei being a creep. Probably. Oh god hopefully...
IF Yumi develops an attachment to MC ?
Ami might just kill if she catches sensei in the act with a girl, Makoto especially.
Maya would probably carry out a scientific experiement titles " In how many pieces can you chop up a Noriko ? "
Noriko will 100% kill Kirin is she figures out that she is crushing on sensei.
Drunken Sana might have a few things to say about her Sensei being stolen, especially after how much she opens up to him.
...
The more I type, the more I feel like act 2 will be a battle rolaye.
Rin could actually be in that list as well if Sensei really hurts Futaba, same reason for Yumi/Chika dynamic. Rin is super-chill and attached to Sensei as well but still, that's her red line.
In a weird, similar fashion I'd picture Ayane becoming protective of Sana as well. She would hate herself and Sensei for that, but she would protect Sana over her attachment to Sensei. Not cause she wants but she is genuinely a good person.
That aside, can you guys please not picture my favorite character as the upcoming Star in "Japan Chrime Series"? Would be nice. I am fully aware that she will end in a blender, or she will put everyone else in a blender, but I am willing to pretend otherwise for as long as possible. ^^