The original intention and direction of school social work development is prevention and development. Analysis based on in-depth interviews and agency service records shows that Shanghai has established a judicial-oriented school social work service model based on the tertiary prevention of crime, and has made remarkable achievements in curbing juvenile delinquency and assisting school development. However, Shanghai also faces many practical difficulties, including the lack of effective connection between the tertiary prevention, the difficulty of breaking double barriers (of administration and technic), and the weak assessment ability of first-line social workers. Such difficulties affect the function of prevention and development of school social work. In the future, Shanghai should seize the opportunity of the promulgation and implementation of the law on the protection of minors and schools, take prevention and development as the orientation, and start from the dimensions of individuals and schools, basic needs and developmental needs to reconstruct the school social work service framework. Here, we should focus on general prevention, promote the internal connection and cohesion between the three levels of service, and pay attention to improving the professional ability of first-line school social workers.