The Columbine High School massacre, commonly referred to as Columbine, was a school shooting and attempted bombing that occurred on April 20, 1999, at Columbine High School in Columbine, Colorado, United States. The perpetrators, twelfth-grade students Eric Harris and Dylan Klebold, murdered twelve students and one teacher. Ten of the twelve students killed were in the school library, where Harris and Klebold subsequently died by suicide. Twenty-one additional people were injured by gunshots, and gunfire was also exchanged with the police. Another three people were injured trying to escape. The Columbine massacre was the deadliest mass shooting at a K-12 school in U.S. history, until it was surpassed by the Sandy Hook Elementary School shooting in December 2012, and later the Uvalde school shooting in May 2022, and the deadliest mass shooting at a high school in U.S. history until the Parkland high school shooting in February 2018. Columbine still remains both the deadliest mass shooting and the deadliest school shooting to occur in the U.S. state of Colorado.

A zero-tolerance policy in schools is a policy of strict enforcement of school rules against behaviors or the possession of items deemed undesirable. In schools, common zero-tolerance policies concern physical altercations, as well as the possession or use of illicit drugs or weapons. Students, and sometimes staff, parents, and other visitors, who possess a banned item for any reason are always punished. Public criticism against such policies has arisen because of the punishments the schools mete out when students break the rules in ignorance, by accident, or under extenuating circumstances. The policies have also been criticized for their connection to educational inequality in the United States.