This article focuses on the use of a value-added approach for promoting school improvement. It presents yearly value-added estimates, analyses their stability over time, and discusses the contribution of this methodological approach for promoting school improvement programmes in the Portuguese system of evaluation. The value-added model is applied to longitudinal data gathered from a representative sample of students enrolled in primary school, using mathematics scores as outcome variable. The estimates are used to determine a systematic positive or negative school value-added effect. Results indicate that at least 41% of schools remain at the same level over 3 years, 15% present an instable pattern, and approximately 40% show a positive or negative trend. It is argued that these results, especially those that reveal systematically underperforming schools, could have relevant implications for educational policy geared towards school improvement.