This chapter certainly escalated things. I think Kudou's SA attempt on Kurumi will get interrupted. The author purposely kept Naoi's location hidden for one, so she might actually be pretty close to Kurumi's current location. Plus, it's the same woods that Naoi lured Kudou into in chapter 20.5, meaning it's an area Naoi is very familiar with. As for Kurumi, she isn't sick this time and the person trying to SA her isn't her childhood friend, so she could end up reacting very violently once her initial shock wears off.
How things play out greatly depends on what the author intends for Kudou and the class in general, though Kudou escalating her behavior to this level pretty much spells the end of her run as the antagonist, at least for the near future. Naoi knows attacking Kudou won't help, as it's been Kudou's goal the entire time. So, Naoi could do something unexpected and call Kudou's mom to the scene to catch Kudou in the act of doing something awful. Chapter 20.5 revealed how Kudou feels disconnected from her family, so forcing them to see she needs help could be her turning point. In this case, they could pull her from Naoi's school, and it's perfect timing, since they could use the accusations against Horie-sensei as a pretext to not draw attention.
Alternatively, if the story wants to go a bit darker, Kurumi could take Naoi's advice from chapter 3 and use a nearby object as a weapon against Kudou. This could result in her cracking Kudou in the head with a rock or something, leaving her severely injured. The location they're in now is extremely isolated, so Kurumi could get away with this, especially if Naoi arrives soon after to help her. So many possibilities here. It being the same location where Naoi triggered Kudou's obsession with her in the first place means it's most likely meant to be the resolution to Kudou's obsession. Either way, the current arc seems to be ending with both Horie-sensei and Kudou on their way out of the door. And with everything that's happened, Naoi won't be able to hide her history with Kudou from Kurumi anymore.
As for the teacher, it looks like the situation with Mine went alot deeper than most people thought. Considering the start of the chapter revealed that Mine's fixation on Isanuma began in middle school, it's pretty clear that she set him up from the start. Either he did something to placate her, like the date with Isanuma, or he really did try to have a relationship with her. Considering the chapter points out that he was more worried about going against Mine than the photo of him hugging Isanuma, it's definitely something more than a hug on that video.
So, Mine's plan to manipulate everyone seems to be working, at least for now. Though she could get herself into trouble again if she targets Kurumi, Naoi, or Azuma. I consider these 3 the most likely ones to realize her story is bogus. The teacher was only her pawn. She'll likely slip up again if Isanuma doesn't become isolated enough for her liking, or if Isanuma falls for someone else now that she hates Horie-sensei.
I think the female staff member that went to talk to the class will be their new teacher. I think she might be the P.E. teacher from the dodgeball leaflet in chapter 10.5. Having someone new in charge of the class could certainly shake the classroom dynamics up, as Horie kept dodging his responsibilities, especially toward Naoi and other students who actually needed help and guidance. Though I have to wonder if Kurumi might get into some trouble here, since the school now knows that Horie was hiding the previous blowups in the class, and the whole class heard Kurumi confess to breaking the class photo and carving up Isanuma's desk on the day she slapped her.
Only 2 chapters left in this volume, so some major things are definitely going to happen. Looking forward to next chapter.