School shooters aren't often serial killers because serial killers have what is known as cooling-off times, or times when they don't kill anyone. They are instead classified as spree killers or killers who kill enough people in a short span of time.

This picture, for those curious, is one of the Columbine shooters, Dylan Klebold. For an easier time, please reference the image on the top right. https://www.telegraph.co.uk/news/worldnews/northamerica/usa/12155354/Mother-of-Columbine-shooter-Dylan-Klebold-says-she-thinks-of-victims-daily.html