This paper aims at extending the understanding of principal leadership in support of teachers' development of robot-based pedagogy for students in a cross-school professional learning community (PLC) of Hong Kong's special education. Sixty representative teachers from over 30 special schools, that is 50% of the total number of special schools, participated in the community over the past 2 years. The teachers not only developed the pedagogy but also observed and collaborated in the schools regarding their technological, pedagogical and content knowledge. The development of robot-based learning for individual students' different special educational needs is an innovative differentiated approach to fostering socially just learning in schools. The key research question is: How could teacher professional development of robot-based pedagogies for all students build a social-justice school? We adopted a qualitative cross-case study of four schools participating in the PLC. Although the professional development did not aim at principals' and teachers' social-justice practices for all students, the case study demonstrated the principals' leadership support and the teachers' enthusiasm for learning to change their pedagogies with the use of new robotic technology for socially just differentiated teaching. Implications of the cross-school PLC for building social-justice school communities are also discussed.