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 (if the policy is followed) 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.

In the United States and Canada, zero-tolerance policies have been adopted in various schools and other educational platforms. Zero-tolerance policies in the United States became widespread in 1994, after federal legislation would withhold all federal funding from states that did not expel students for one year if they bring a firearm to school.

The zero-tolerance term began with the Gun-Free Schools Act of 1994, when Congress made federal funding of public schools conditional upon the adoption of zero-tolerance policies for firearm possession. Similar policies of intolerance, coupled with expulsions for less serious behaviors than bringing a weapon to school, had long been a part of private, and particularly religious, schools.

The use of zero-tolerance policies in public schools increased dramatically after the Columbine High School massacre in 1999, with principals declaring that safety concerns made them want zero-tolerance for weapons. These rules led to disproportionate responses to minor or technical transgressions. Cases that attracted international media attention cases include students being suspended or expelled for offenses such as possession of over-the-counter and/or prescription drugs on campus with the permission of the students’ parents, keeping various pocketknives in cars, and carrying woodworking tools outside of a wood shop classroom. In Seal v. Morgan, a Tennessee student was expelled because someone else's knife was found in his car on school property, despite his protestations that he was unaware of the knife's presence; in 2000, the courts struck down the expulsion as having no rational basis. In some jurisdictions, zero-tolerance policies have come into conflict with freedom of religion rules already in place allowing students to carry, for example, kirpans.

In the "kids for cash" scandal, judge Mark Ciavarella, who promoted a platform of zero-tolerance, received kickbacks for constructing a private prison that housed juvenile offenders, and then proceeded to fill the prison by sentencing children to extended stays in juvenile detention for offenses as minimal as mocking a principal on Myspace, scuffles in hallways, trespassing in a vacant building, and shoplifting DVDs from Walmart. Critics of zero-tolerance policies argue that harsh punishments for minor offenses are normalized. The documentary Kids for Cash, interviews experts on adolescent behavior, who argue that the zero-tolerance model has become a dominant approach to policing juvenile offenses after the Columbine shooting.

In New York City, Carmen Fariña, head of the New York City Department of Education, restricted school suspension by principals in 2015. The Los Angeles Unified school board, responsible for educating 700,000 students, voted in 2013 to ban suspensions for "willful defiance", which had mostly been used against students from racial minorities. A year later, the same school district decided to decriminalize school discipline so that minor offenses would be referred to school staff rather than prosecuted. The previous approach had resulted in black students being six times more likely to be arrested or given a ticket than white students. The district saw suspensions drop by 53%, and graduation rates rise by 12%.

Media attention has proven embarrassing to school officials, resulting in changes to state laws as well as to local school policies. One school board member gave this reason for changes his district made to their rigid policy: "We are doing this because we got egg on our face."

There is no concrete evidence that zero-tolerance reduces violence or drug abuse by students. Furthermore, school suspension and expulsion result in a number of negative outcomes for both schools and students. The American Bar Association finds that the evidence indicates that minority children are the most likely to suffer the negative consequences of zero-tolerance policies. Analysis of the suspension rate of students shows that black females and other racial minorities are suspended at a greater rate. The American Psychological Association concluded that the available evidence does not support the use of zero-tolerance policies as defined and implemented, that there is a clear need to modify such policies, and that the policies create a number of unintended negative consequences, including making schools "less safe".