This research evaluates the role of school administrative measures (including creating a greater school belonging and paying attention to student attendance) as independent variables and their resulting core competencies through the mediator of students' experience with school bullying. This study adopted a multi-level mediation model to empirically analyze data from middle school students and school administration in four Chinese provinces based on the 2015 Programme for International Student Assessment (PISA). Data were collected from a total of 9,060 students and 260 administrative staff. The results were: (i) Relational bullying was significantly and negatively correlated with the three core competencies, although no significant impact was found for either verbal or physical bullying; (ii) Schools which were successful in creating a more positive environment, including greater school belonging and greater attention to students' attendance, demonstrated lower levels of relational bullying; (iii) In terms of school-level variables, a greater sense of shared belonging had a direct effect on improving student performance on math and science competencies, while greater attention to students' attendance was associated with higher student scores on all three core competencies; (iv) Furthermore, school-level variables, including the sense of shared belonging and greater attention to students' attendance demonstrated a positive indirect effect on students' core competencies through the mediating effect of reduced relational bullying.