Is there any mentally unstable person willing to replay the game while taking notes on Ami, when she changes personality or has some suspicious dialog?
I have been replaying the game and am in chapter 2. Several moments in chapter 1 comes to mind.
1) At the very beginning of the game, Ami very easily accepts Sensei's lost memory. I thought it was Reset magic or writing convenience. But Ayane freaks out much more than Ami. Ami does not show any concern for this.
2) Ami tells Sensei about his notes about students so he can remember their names. Except Sensei never took these notes. Maya made them and put them on his desk. When they reached the home, Ami also says the key is missing and the lock needs to be replaced. Her snarky comment implied she knows it was Maya.
3) Before Sensei went to class, Ami quizzed Sensei about everyone's names. She said things are gonna go wrong if he can't remember. (Like she experienced them before)
4) When the three of them went to cafe first time for Ami's birthday, she went full incest-gremlin on the waitress. Sensei stopped her. She told Sensei that they wouldn't actually get in trouble since she knew what to say so things don't escalate.
So, I think Ami knew from the start. Most forum users think that if Ami hid that knowledge that means she was acting for most parts. I think Ami's reactions and her emotions are genuine. We will probably have to wait to get why Ami hates Maya and seemingly ok with rap*ing her. I don't believe Ami went full murder hobo and murdered everyone. I think Maya lied to Ayane about it.
PS: Stuff is from my memory. I am too lazy to screenshot and add images from the game.