This paper advocates the use of the Dynamic Approach (DA) to promote Student Self-Assessment (SSA) for formative purposes. DA is based on the assumption that professional development should be differentiated to meet teachers' needs, while engaging them into guided critical reflection. A sample of 45 teachers was randomly allocated into two groups: one experimental group which received an INSET course based on the DA and one control group. Assessment skills and student outcomes (cognitive and metacognitive) were measured at the beginning and the end of the intervention. Teachers employing the DA improved their skills but no change in the assessment skills of the control group was observed. DA had an impact on student achievement in mathematics. Implications are drawn.