Reduced class sizes have a positive impact on pupil achievement and classroom processes. Pupils receive more individual attention and are better behaved and more on-task in small classes. In Hong Kong, small-class teaching has become a major government policy in primary schools. It is being implemented in phases, beginning in the 2009–2010 school year in Primary 1. Research shows that the benefits of a small-class environment are not automatic if teachers do not make changes to their teaching practice; hence, various types of campus-based professional development programmes have been organised for teachers to adapt their teaching in the small-class environments for the purposes of optimising the student learning. However, teachers tend to revert to their usual teaching practice soon after undertaking the relevant professional development training. School support is essential to provide encouragement for continuous improvement in order to sustain the effect of the campus-based professional development programmes. This article begins with an overview of the research on class size, then describes the implementation of small-class teaching policy in Hong Kong, compares the campus-based with school-based professional development programmes in small-class teaching, analyses the weakness of the existing school-based professional development programmes and finally argues that a self-owned model of school-based teacher development can be effective in sustaining the effect of the professional development programmes that teachers have undertaken.