This paper, through discussion of a teaching intervention at two secondary schools in Hong Kong, demonstrates the learning advancement brought about by group work and dissects the facilitating role of teachers in collaborative discussions. One-hundred and fifty-two Secondary Two (Grade 8) students were divided into three pedagogical groups, namely "whole-class teaching", "self-directed group work" and "teacher-supported group work" groups, and engaged in peer-review, team debate, group presentation and reflection tasks related to a junior secondary science topic (i.e. current electricity). Pre- and post-tests were performed to evaluate students' scientific conceptions, alongside collected written responses and audio-recorded discussions. The results indicate that students achieved greater cognitive growth when they engaged in cooperative learning activities, the interactive and multi-sided argumentative nature of which is considered to apply particularly well to science education and Vygotsky's zone of proximal development framework. Group work learning is also found to be most effective when teachers play a role in navigating students during the joint construction of conceptual knowledge.