This paper investigates the sustainability and the impact of offering the dynamic approach (DA) to schools for more than 1 year in improving student achievement in mathematics (quality) and reducing the impact of socioeconomic status (SES) on achievement (equity). A sample of 56 schools in socially disadvantaged areas in three countries (i.e., Cyprus, Greece, and Ireland) and their Grade 4 and 5 students (n = 2,844) participated in this study. Two experimental groups used DA to develop school improvement strategies and action plans. The first experimental group, which employed DA for only 1 school year, was more effective than the control group in promoting student achievement in mathematics and reducing the impact of SES on achievement both at the end of the 1st and 2nd implementation year. Schools which made use of DA for 2 years were found to be the most effective at the end of the 2nd year.