A crushing depiction of the pain children inflict on each other through bullying. This is seen with Shouko being bullied by Shoyo in the early part of the movie.
Those who stand by and watch the pain being inflicted on others which was the case of most of students. Miyoko was a glaring example of this.
The second thing which was painful to watch was the struggle shouko was having to go through just to gain the acceptance of people she wanted as friends, starting with Shoya. You see her being shunned when she wanted to join in conversations with her classmates. Even when she had gained some friendships a few years later, she still felt like she was a drag and a burden on the lives of her friends and family. The end result was a suicide attempt.
The part true friends play in redemption was also in full display between Shoya and Nagatsuka. Sometimes just the acceptance of one true friend is enough for complete emotional healing.
In a nutshell, this movie has something to teach everyone about life, love, friendship, forgiveness and acceptance. In my opinion students should be shown this movie at all levels where they will be able to understand what this movie is teaching.
The third thing has to be the beauty of a changed person and crushing realization that society still judges him/her by their past actions. Shoya went through a redemptive phase but most people around him refused to accept him for the person he had become. He believed then that he had no place in the world hence his opting for suicide as an escape route.