This paper presents the results of a European study investigating the extent to which the Dynamic Approach to School Improvement (DASI) can help schools situated in socially disadvantaged areas to improve their effectiveness. At the beginning of the school year 2015–2016, a sample of 72 primary schools in four European countries (Cyprus, England, Greece and Ireland) was randomly allocated into the experimental and control groups. A questionnaire measuring the functioning of school factors related with the school learning environment, school policy for teaching and school evaluation was administered to all teachers of the school sample (n = 762). A battery of mathematics tests and a questionnaire measuring students’ socioeconomic status (SES) were administered to all students of grades 4–6 of the school sample (n = 5560). The experimental group made use of DASI to develop improvement strategies and action plans. Feedback was provided to the control group regarding their students’ achievement and the functioning of school factors in their school. Ιn each country, DASI had an effect on promoting student learning outcomes. For the control group of each country, the total effect of SES on student achievement at the end of the intervention was bigger than the effect of SES at the beginning of the intervention. No increase in the effect of SES was identified in the schools of the experimental group. Implications of findings for establishing a theory-driven and evidence-based approach to improve the quality and the equity dimensions of school effectiveness are discussed and suggestions for future studies are provided.