'Penis logic' 'I want you, so it's right'
Many here insightfully deduced that the whole thought experiment was really the high school teacher's way of convincing his student she should keep screwing him.
That doesn't mean it wasn't a little fun for the ride, maybe not how you'd do it, but it held its own internal consistency, for awhile.
Things went wrong when good story-telling became self-serving apologist b.s. For this, I wasn't surprised it was written and directed by the same person. He must have had in his head this idea of a hot young girl that he wanted to control and wrest away from some guy who he deemed "unworthy", losing in that respect you see the director's touch at the end of the film where we see three versions of the teacher leaving, being 3 options to being rejected, let it go and eat a sandwich, kill himself, or just keep dreaming of her, which was the chosen path, hence the film and the faulty logic in the story. You could almost taste the b.s. of Petra's character in the air, the writer-director loved her so much she's portrayed as a Jesus like figure by the end where every other character would happily kill him/herself to save her.
See the first two iterations were the teacher saying "choose the logical course" but it also revealed his own insecurities and flaws. He's possessive. In each story he chooses to kill everyone because he doesn't get what he wants. Something he was doing in the classroom by manipulating his students and threatening them if they didn't go along with him.
I guess we're to assume the 'perfect girl' figured this out from the get go and that's why she tried to leave, it is also why her iteration is to throw everything in his face.
Where all of this goes off the rails is the conclusions the characters make. The teacher, being a manifestation of the writer-director (or at least the sympathetic unsung hero) creates scenarios of extreme stress and suffering for the sake of his "choose me" logic. He ends his story with mass rape and oppression as a solution because of his "by any means necessary" point.
What could have been a brilliant counter-point was Petra's version. Many here think she was appearing to be insightful, but led to nothing, or worse was making some kind of flippant point about temporary pleasures superseding logic.. Well she was and she wasn't. She was making a brilliant argument for the lesser characters up until the point where she lead them to a ludicrous conclusion of killing themselves on an island. The smart conclusion would be that the stress-free love filled environment she created helped the characters procreate as the stress-filled versions couldn't. They would prosper through love and caring about each other's well-being. They weren't all experts, but there was a genius, there was a person with perfect memory, and there was an f-ing engineer. If Polynesians could travel the Pacific on rafts 2,000 years ago, I think they could have done just fine. The teacher i.e. the writer-director was wrong.
This I believe is entirely because the filmmaker doesn't even know the right answer, and was probably still clinging to that uber-logic (that somehow emanated from the teacher's teen screwing penis) His idea of this lovely girl was that she was tossing aside his eternal logical love and future for a fling with a boy, and so she must be the kind of person who would happily die on a beach. The write-director, and the teacher never learned how wrong they are, so the film comes off disjointed and confusing in this regard. The writer-director attempted to show Petra's version in a sympathetic, yet simultaneously condemning light. So you could feel comfortable saying "that's beautiful, but unrealistic". When it's precisely wrong, it's his idea of banging beautiful students and manipulating them to his will that is unrealistic, or at least wrong and unsustainable.
The unreliable narrator. Or in this case, writer-director. Just as flawed as the teacher in the film. Steering "logic" towards his sexual desires.
The ends destroy the film. The film opens with two young people in love, and continues with some intelligence as we see both of them use their minds to challenge the teacher and succeed. Yet by the end of the film nothing but a bad taste as every scene and point about their love is destroyed by both Petra and the teacher, as she clearly enjoys banging him too, and apparently likes keeping this guy on the side. They go off to college together, she'll be banging teachers there on the reg too. Kinda the reverse of a love story. It begins all loving and cherishing. It continues with people fighting for their love, and then it ends disjointed, lost, remorseful, with a liar choosing the patsy over the manipulator.