Well, the development of a sense of superiority among the 'Wave' members over non - members is an important event. This causes division in the school and also starts to show the darker side of such extreme groupthink. The students begin to exclude others and enforce the 'Wave' ideology more strictly, which ultimately leads to chaos and self - reflection.
The climax of the story in the film is when the teacher realizes the extent of what he has created. The students have become so immersed in the 'Wave' that they start to exhibit extreme behavior. For example, they begin to harass those who oppose the 'Wave'. This makes the teacher understand that he has to put an end to this experiment gone wrong, and he finally does, leading to a moment of realization for all involved.
In 'The Wave' film based on a true story, one key event is the teacher's experiment in the classroom to show how easily a group can conform to a collective ideology. He starts simple rules which gradually lead to a more extreme sense of unity and obedience within the group.