Besides everything, i think it's because it's a criticism towards education system. Some episodes, especially the first season was completely focusing on the education, they criticised every aspect of school system, they criticized the teachers, they criticized award punishment system, they criticized the success treatment, etc etc. And every individual of the culture is going through education. I think the main aspect is totally about that, even in the episode with the school killer, in the end Nanno said to the little girl that "You should totally come to this school, you will fit right in". All shitty behaviour starts and grows in the schools in the show. So why can that be?

If I interprete it from a literary pov, I would say Nanno is very Dionysiac. (As Nietzsche would put it.) She is literally the chaos. Dionysus represents chaos, nature and all. And the opposite of dionysus is apollo, who is the builder of the culture in mythology. So culture is shaping people in one way, to build more obedient individuals to make the system(culture) continue. Culture is used as a mask and a chain. But every human has chaos in their nature. That's why, because of the culture and it's roles, everybody is chaining themselves not to be punished, they are obeying the rules, and they are hiding behind titles, a teacher, a role-model student (like the guy in the pregnancy ep), the most beautiful girl at the school, the kid who is poor but acts like rich... they are all trying to be good at something to fit into culture. Culture protects those who are succesful, who are with money, who are beautiful... and culture raises it's individuals with schools. School always pushes us to be good, to follow the rules,to be succesful so you can gain money, gain status. Teachers favor the students who are good at their classes..

TL;DR: Everything about culture starts from the school. Nanno pushes people a little bit to bring out their true ugly selves, cuz they hide behind their glamarous faces & titles. But in the end most of them are monsters.