TF is wrong with this school and parents?! What?
Manga might exaggerate it, but bullying in Japan is very much treated as a sort of "don't ask, don't tell" kind of deal.
Japanese culture tends to emphasize the values of self-sacrifice for the greater community. Issues like bullying or abuse are treated as private matters to be kept out of the public eye. Confronting bullies risks negatively impacting the "community" that is the student body and faculty. So as a result teachers will often turn a blind eye and leave it to the students to sort it out themselves.
More and more people in Japan are advocating against this sentiment and pushing to take more active action against bullying, so things
are changing. But it's a process, and in the meantime "dealing with bullies while nobody else is willing to do anything" is a depressingly relatable backdrop for a manga story. It's why so many manga start with some kid being bullied and alone, before the escapism aspects of the story to kick in.