The term zero tolerance started in New York that occured many criminals. It means that never forgive any misdemeanor. Actually, in 1990, New York decided to regulate free passengers in subway, and after that, subway criminals are surprisingly decreased half than before.  Zero tolerance is based on broken window theory.  Broken window theory means a broken window makes people think that build leave, so it finally causes many other criminals. After New York’s success, many schools accept Zero Tolerance. For preventing big trouble in school, teachers prohibit bring plastic knife for peeling apples. These days, in Korea, the incidets that students hit friends or even teacher are increasing. In deed, a teacher quited her job because of insolent student who kicked her. At this point of time, people ought to think about what problem is. Zero tolenrance might be rigid rules to young students, but they ought to know how they behavior and what manners is.