This study examines the learning and related impact on classroom teaching of six EFL high school teachers' participation in collective lesson planning discussions (CLPDs) organised by a teaching research officer. Data were gathered from multiple sources, including observation notes, transcripts of collective lesson planning discussions and interviews, questionnaires and teaching materials. Data analysis shows that CLPDs exerted important influences on participating teachers' EFL teaching as a result of sharing resources and exchanging ideas. Some factors were found to impede the implementation of ideas and methods suggested in CLPDs such as teaching environment and students' language proficiency. Further research needs to examine the collaboration of teaching research officers and university researcher on a more constant basis to gauge the effectiveness of teacher development in fostering student achievement. Keywords Collective lesson planning discussion, EFL high school teacher, professional learning community, lesson preparation group, teaching research group