Summer vacation looks like it's leading to major relationship changes all around.
Kurumi's going through serious development. She's being more observant of others and their motives. She's being more assertive in her interactions with other people. And she's experiencing new feelings, like when it looked like she was tearing up when she thought that Naoi stood her up on their date. I think we're getting Naoi's backstory soon since Kurumi saw Naoi's dad abusing her. I guess he really doesn't work if he's shaking down his own daughter for money. Naoi was trying really hard to get away from him so that she could have her date with Kurumi. She was probably worried her dad destroyed her chances and that Kurumi wouldn't give her a second chance after getting stood up once. But luckily fate had other ideas, so now Kurumi shows up to deepen her relationship with Naoi.
As for the plot with Sensei and Isanuma, I'm kind of not 100% sure where it's going. Could Sensei really be dating high school girls? Maybe, but it's also possible the person he was seen with was the red-haired woman from the chapter 11 cover. Maybe she's just someone he doesn't think is socially acceptable to date, because he likes to keep up appearances. But keeping up appearances by agreeing to go on a date with Isanuma is almost certainly going to backfire on him. If the red-haired woman is his girlfriend, she'd dump him for cheating on her, even if he's only agreeing to the date to buy Isanuma's silence about her confrontation with Kurumi in chapter 10. And if they're found out, then he'd almost certainly try to push all of the blame onto Isanuma for suggesting the idea to begin with. He seemed super uncomfortable with her attention this chapter even once Kurumi was out of the room. He's a horrible teacher regardless of what's really going on with his dating life.
Isanuma almost feels like she's getting setup for heatbreak on purpose by Mine. What's Mine's motive? She might be in love with Isanuma and wants to destroy her crush on the teacher. Or maybe even Isanuma's life so that she becomes all Isanuma has left. Kurumi may have just been a convenient pawn in this plot, maybe because Isanuma was jealous of her seeming perfection already. Plus, it would give Isanuma more confidence to do something foolishly reckless by thinking Sensei would accept dating a high schooler. Mine seems to smile around Isanuma when her face is normally hidden. She blushes around her alot too. This might all be a form of twisted love.
And finally Azuma. She's secretly been an important character since chapter 1. There was a ponytail shown when Kurumi was shoplifting. And in chapter 3 Azuma seemed very wary of Naoi. So she might've seen both Kurumi almost shoplifting and Naoi recording the whole thing. In the background we've seen her growing nervousness of Isanuma's increased animosity toward Kurumi. Her increasing isolation from her friends, like when she was left out of karaoke in chapter 8. But she still tried to be a good friend in chapter 10 by trying to stop Isanuma from doing something to get herself in trouble, even though Mine was conveniently nowhere to be seen and did nothing to help. And how Azuma has some choices to make. Mine and quite possibly Isanuma are likely mad at her for telling Sensei about Isanuma bullying Kurumi. And to make things even worse, she finds out that Mine was a manipulative liar and rumor-monger all along. And then that same liar trying to shame her into silence. Only to take yet another blow when Isanuma turns out to be a hypocrite who happily dates Sensei after treating Kurumi like absolute trash for thinking she did the same. Isanuma's animosity was out of jealousy, not because she looked down on the behavior. But there is hope. Mine doesn't realize that Azuma has a hidden Ace up her sleeve if she chooses to use it. To go to Naoi for help. After all, who better to get evidence than the person she saw filming someone shoplifting? Fight fire with fire.