This paper evaluates the effects of grade retention on attainment by exploiting a reform introduced in 2001 in the French-Speaking Community of Belgium whereby the possibility of grade retention in grade 7 was reintroduced. It uses the Synthetic Control Method to identify the best possible pre-treatment control. Data come from three waves of the PISA study (corresponding to periods before and after the reform) that contains test scores of representative samples of 15 year-olds. These are used essentially to answer two questions. First, has the 2001 grade repetition reform at least succeeded at filtering out weaker pupils, pupils who would presumably be disadvantaged by being promoted directly to higher grades. This is a minimum condition for grade retention to be justifiable. Second, do these “treated†students achieve better/worse when they repeat (and attend a lower grade) than when they are “socially promotedâ€. (and attend the age 15 reference grade 10)? We find significant evidence of positive screening but we fail to demonstrate that those filtered out perform differently under the “grade repetitionâ€.regime than under the “social promotionâ€regime.