This paper examines the development of the probation system in the early 20th century, focusing on the activities of probation officers in the juvenile court in the US state of California and their cooperation with urban public schools. The sources used include proceedings of the National Conference of the Charities and Correction (NCCC), annual reports of the City and County of San Francisco Juvenile Court, and publications of the Los Angeles County Juvenile Court. I first address a discussion of the NCCC, showing what issues were shared within the national network of social work at the time. Second, after presenting an overview of the juvenile justice system in California, I describe the“ problem” of truant children who were placed on probation. Third, I explore the cooperation between the juvenile court and public schools, and the functions expected for probation. Through the above analysis, I identify the characteristics of California’s probation system and the activities of probation officers within that system. First, in California, the juvenile court and public schools actively collaborated, as proposed by the NCCC. As stated in the Juvenile Court Law, probation officers had powers equivalent to those of truant officers. Although prior research has tended to focus on probation officers’ interventions in families, there was a close collaboration not only with the home but also with public schools. Second, in California, the definition of dependent children in the Juvenile Court Law of 1903 included“ a persistent truant from school.” It is significant that according to the Juvenile Court Law, children absent from school were considered subjects for protection. Third, the juvenile court attempted to prevent and solve crimes; that is, to educate children by using the probation system, rather than merely punishing them.