["Criminology, Volume 61, Issue 1, Page 40-73, February 2023. ", "\nAbstract\nIn a quasi‐experiment, we examine whether changing schools during the transition from 8th to 9th grade influences adolescent delinquency, using a sample of more than 14,000 students in 26 public school districts (PROSPER study). The dataset follows students for eight waves from 6th through 12th grade and facilitates a unique, direct comparison of students who change schools with those who remain in the same school during this period. Results show that students who transition between schools report significantly less delinquency after the shift than those who do not, and that this difference persists through 10th grade. This decline is most pronounced when adolescents from multiple middle schools move to a single high school (i.e., multifeeder transitions). Students who transition between schools have fewer delinquent friends and participate in less unstructured socializing following the change in school environment, which partially mediates their reduced delinquency. Results provide some support for theories of differential association and routine activities. Our findings highlight the role of a crucial, yet understudied, life transition in shaping adolescent delinquency. The results from this quasi‐experiment underscore the potential of alterations in social context to significantly dampen juvenile delinquency throughout high school.\n"]