In the literature that investigates the causal effects of education on outcomes such as health, wages and crime, it is customary to measure education with years of schooling, and identify these effects using the exogenous variation provided by school reforms increasing compulsory education. If the reforms are correlated to changes in school quality, and school quality is an omitted variable, this identification strategy may fail. We develop a new test that uses the information provided by two distinct measures of cognitive skills and show on PIACC and IALS data that the test cannot reject the internal validity of this popular identification strategy.