A zero-tolerance policy in schools is a policy of punishing any infraction of a rule, regardless of accidental mistakes, ignorance, or extenuating circumstances. In schools, common zero-tolerance policies concern 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) to be punished.
In the United States and Canada, zero-tolerance policies have been adopted in various schools and other education venues. Zero-tolerance policies in the United States became widespread in 1994, after federal legislation required states to expel any student who brought a firearm to school for one year, or lose all federal funding. More: en.wikipedia.org.