This paper explores how organizational features of high schools influence the informal interaction of students attending them. Size of school, composition of the student body, and differentiation into curriculum and grade levels are factors which affect the patterns of informal interaction among students. The informal social arrangement of students within schools is viewed as a social network in which the friendship and esteem relationships are particular ties. Properties of social networks (reachability, cohesion and participation) are defined, and actual measurments of 40 social networks are derived and studied in connection with specific organizational properties of the school. (Author)