Social control and procedural justice theories indicate that informal social control reduces problem behaviors. However, many schools have implemented formal control mechanisms such as school security measures. This study examines the association between school security measures (security personnel, metal detectors, and surveillance cameras) and students' perceptions of informal social control (relationships with teachers, other school adults, and the fairness and consistency of school rules). We used structural equation modeling to examine these relationships in a nationally representative sample of 6,547 secondary students surveyed as part of the School Crime Supplement to the National Crime Victimization Survey (M age = 14.94; 51% male, 60% White non-Hispanic, 14% Black non-Hispanic, 20% Hispanic). The results indicated that the presence of security personnel in schools was associated with poorer student relationships with teachers. Findings for the other school security measures were nonsignificant or inconsistent across models. Implications for theory and practice are discussed.