There is a broad consensus on the impact of teacher quality on students' outcomes. However, the debate on how to evaluate the impact of teacher training on student improvement remains open. The evaluation of the impact of in-service teacher training, organized in a network for different schools, has been analyzed very little to date. Our research displays an innovative approach in this regard, through an In-Service Professional Development Program based upon scientific evidence and dialogic principles: The Pedagogical Gatherings 'On Giant's Shoulders'. We conducted a multilevel communicative study to analyze its impact upon students' achievement and schools' outcomes whose teachers taking part of the Gatherings. Our contribution provides an advancement in the analysis of educational impact in teacher training. We provide indicators to identify those training programs that improve educational outcomes, according to the United Nations Sustainable Development Goal number 4: quality education for all.