Psychopathic and Creepy vibes are given off in the show, however the mood sometimes jumps to lighthearted and back.
A girl gets the word "Die" written repeatedly on her locker
One of the main characters gets bullied frequently
This anime is full of very disturbing themes including rape, self harm, pedophilia, abuse, sexual harassment and other topics discussed by young 12-13 year olds (which can be very upsetting for some viewers). Although very little is shown, it's clearly hinted at and talked about in conversations between the characters.
self harm is shown and mentioned several times throughout the series
It's heavily implied that Rika self-harms throughout the season