This study used border theory to understand the learning experiences of student teachers transitioning from campus-based learning to classroom teaching. For teacher candidates learning to teach English as a second language in primary schools in Hong Kong, crossing institutional borders during practicums was highly complex, due to the conflicting expectations, agendas and cultures of university classrooms and elementary schools. School-based mentors played an important role in maintaining or dismantling institutional borders, thus enabling or constraining knowledge transfer across domains. The findings may help educators to create a third space in which student teachers can connect campus-based learning to school practice. [Copyright of Teaching & Teacher Education is the property of Elsevier Ltd.]