Rated TV-14 for mild violence and mild language.
Most students have a backstory explaining their personality and behavior in the show. Some, if not most, of those backstories may be disturbing (suicide, bullying, depression, solitude, stealing, and many more).
Some callous and reckless people are shown in the show. It may be frustrating to watch these characters behave however they want without repercussions.
Despite the show being an anime and sometimes lighthearted, it is very realistic in portraying people's personalities and weaknesses. All characters are absolutely different from each other, and many portray traits of trauma or personality disorders.