This paper presents a new method that goes beyond the measurement of average value-added of schools by measuring whether schools mitigate or intensify grades dispersion among initially similar students. In practice, school value-added is estimated at different levels of final achieve- ments’ distribution by quantile regressions with school specific fixed effects. This method is applied using exhaustive data of the 2015 French high-school diploma and controlling for initial achievements and socio-economic background. Results suggest that almost one-sixth of the high schools significantly reduce, or on the contrary increase, the dispersion in final grades which were expected given the initial characteristics of their intake.