This study aims to examine the effect of accreditation on schools in terms of open system elements. For this purpose, the research was designed as a multiple-case study. Three private primary schools accredited by International Baccalaureate Organization's Primary Years Program (PYP) were chosen as cases. The data were collected through interviews, observations, and documents. The data were analyzed using Katz and Kahn's Open System Theory with a thematic analysis method. Although the three schools have different characteristics, it has been observed that the PYP has similar and positive effects on the system elements. The formation of these positive effects and the successful implementation of the program was observed to be dependent on the attitude of the school community towards the accreditation program. The findings also suggested that the professional competence of the staff as PYP practitioners, the adaptation of change, communication, and team skills played a decisive role. Teachers and administrators working in PYP schools reported that they experienced tension in aligning the national curriculum with the PYP and balancing academic success expectations with the program's achievements. To cope with such tensions, it is argued that schools that want to be accredited need to analyze their realities in terms of the school vision, economic conditions, and parent expectations. It is suggested that schools will benefit better and manage the accreditation process successfully when relevant evaluations, before the start of accreditation studies, are performed on sustainability, cost-benefit, and readiness of the school community.