This study explores misperceptions about peer norms in support of telling authorities about weapons at school and whether perceptions predict personal support for telling authorities. Anonymous surveys were conducted among youth in 30 schools (Grades 5-12) in six states (n = 12,903) between 2006 and 2014. Personal support for telling principals, teachers/counselors, police/security, and parents, separately, about weapons at school was measured as well as perception of peers' support for telling authorities. Most students in most schools personally thought that students should tell authorities about weapons. However, depending on the authority, 37% to 52% of students mistakenly believed that most students did not support students telling authorities. Regression analysis revealed that perceiving support for telling authorities as normative among peers was highly predictive of personal support--much more so than other predictors. Thus, misperceptions of the peer norm as a code of silence may contribute to the problem of weapons at school.