An uneven collection of stories detailing the events leading to a school shooting and exploring the psychology behind the shooter, Kirby Matheson. Unfortunately, the least successful of the bunch (including one from the perspective of the gun itself as it opines about purpose and morality while making the shooter sound like some put upon martyr) drag everything down to where the better ones can't salvage the tone. Some stories are only the most tangientally related, and the ones that focus on the victims keep them passive or, worse, tonally indebted to the shooter for not killing them.
If you are interested in the collection, the two that produce a more nuanced view of it, I would recommend Beth Revis' tale about the boy who was once rescued from bullying by Kirby and reconsiders the act as maybe something selfless and self-seeking, and Courtney Summer's from the perspective of one of Kirby's tormentors who survives and deals with the complicated fallout while maintaining his previous characterization as a monstrous asshole but fleshed out in motivations.
Otherwise I would recommend Hate List or Silent Alarm for stories that deal with this unfortunate phenomenon. They focus on one perspective but show a more in depth view to all parties involved than the mixed bag of this collection.