Each chapter escalates in a way that is so terrifying and exciting! I feel real bad for Hina though. She seems to really like Ai and because of that she tolerates her twisted "affection," but it's quite clear to me that she isn't actually a masochist and that she doesn't enjoy the ... "games" like Ai does. And Ai is just,,,, oof .... piercings are a huge squeak for me and that chapter genuinely had my heart racing and then in the next she straight up chokes her and holds her out a window to get her to piss in fear??? Fucking wow. I'm really enjoying this manga and I am terrified to see how it continues to go. I won't be surprised if the manga takes a dark turn and ends with Ai pushing Hina too far and resulting in a permanent injury.
The "best-yuri" case scenario I can see is that Ai realizes she's doing wrong and starts to see Hina as a human and stop with her extreme sadism (but maybe keeping consensual not-dangerous games).
The "worst-best" case would be Airi pushing Hina so hard she ends up injured and maybe disabled (permanent or not) and Airi feeling guilty and taking care of her.
The "real best" case would be Hina realizing Airi is disgusting and leaving her.
And obviously, the "worst" case would be both girls dying from a dumb game (uncontrolled electroshock for example).
Other options include: Hina is really a masochist but the author doesn't want to show that side of her; Airi being caught by teachers and expelled; or Hina being blackmailed into leaving Airi.