for school violence. Communities with high rates of crime and drug use teach youth the violent behaviors that are carried into schools. Children in violent neighborhoods tend to perceive that their communities are risky, and that these feelings of vulnerability carry over to the school environment. Dilapidated housing in the neighbourhood of the school has been found to be associated with school violence. Teacher assault was more likely to occur in schools located in high-crime neighbourhoods. Exposure to deviant peers is a risk factor for high levels of aggressivity. Research has shown that poverty and high population densities are associated with higher rates of school violence. Controlled longitudinal research indicates that children's exposure to community violence during the early elementary school years increases the risk of aggression later in elementary school, as reported by teachers and classmates. Other, well controlled longitudinal research that utilized propensity score matching indicates that exposure to gun violence in early adolescence is related to the initiation of serious physical violence in later adolescence. Neighbourhood gangs are thought to contribute to dangerous school environments. Gangs use the social environment of the school to recruit members and interact with opposing groups, with gang violence carrying over from neighbourhoods into some schools. Alternatively, many children who grow up in violent neighborhoods learn to deliberately find and make "street-oriented" friends as an instrumental tactic used to avoid being victimized. Without the threat of violence, children more commonly develop friendships based on homophily, or shared traits. School environment Recent research has linked the school environment to school violence. Teacher assaults are associated with a higher percentage male faculty, a higher proportion of male students, and a higher proportion of students receiving free or reduced cost lunch (an indicator of poverty). In general, a large male population, higher grade levels, a history of high levels of disciplinary problems in the school, high student to teacher ratios, and an urban location are related to violence in schools. In students, academic performance is inversely related to antisocial conduct. The research by Hirschi and others, cited above in the section on the home environment, is also consistent with the view that lack of attachment to school is associated with increased risk of antisocial conduct. Prevention and intervention The goal of prevention and intervention strategies is to stop school violence from occurring. According to the CDC, there are at least four levels at which violence-prevention