This is the most unrealistic realistic work, I have to say!!
For the realistic part, bullying is a common phenomenon in schools. As they grow up, most bullies remain unchanged: they either become hypocrites like Kawai, blame the victim like Ueno, or are unable to feel the tiniest flicker of remorse unless they themselves experience the same bullying and isolation, as Shoya did.
But for the unrealistic part, nobody would go as far as Shoya did to redeem himself and make up for his mistakes, and nobody would be as kind-hearted as Shoko was-to the point that she quickly forgave not only Shoya but also the people around her. And that's probably why, even with more intolerable characters, strained relationships, and unfair treatments in the series, people still love this work as long as there is a bittersweet yet fulfilling ending for Shoya and Shoko.