The Movie is about a thought experiment to help someone make a decision
I believe the movie is about Eric the teacher wanting to set up a thought experiment to 'help' Petra to decide the 'right' path she could choose to take after graduation that also includes which guy she would choose more worthy to be with. So the thought experiment is a way to help her make that decision with more insight from other people including him to make that choice.
The teacher puts in his insights about the real world that forms the rules of the experiment, and then he lets the young students run the experiment. However, he is seen to meddle too much with the thought experiment.
The teacher keep killing off the poet from influencing the girl's character with romance. He also tried to lower the worthiness of her boyfriend.
For 2 times without the leadership of Petra, the young students led the experiment to one that led all of the people in it to die. I think this could be interpreted by the teacher(and perhaps also the viewer) that the young students were still not mature enough to make the 'correct' choices in life.
However, at the last try, the supposed smartest student Petra takes hold of the decision making and makes her 'mature' choices.
Because Petra still did not choose reason over her feelings that also reflects her choice to choose her current boyfriend over the teacher, the teacher being biased as he is, saw it as a failed experiment in reasoning and lowered her grade.
Petra was of course allowed by the teacher to decide for the last round because he wishes to see her be unhappy and show that she is still immature and not capable of making the right adult decisions, but he did not expect her to be so mature and strong in her decision making and that she would get to be happy in such a scenario.
I think the message of the movie is that one has to look deeply into people to discover their true inner worth, and not just judge them based on superficial appearances. It also tells one to look for what makes one truly happy rather than attend to the superficial things in life. Petra looked deeply into people as well as her own need for happiness, and saw that her current boyfriend is more compatible or 'worthy' for her compared to the teacher who seem immature or less 'worthy'.
What do you think?