This paper examines the hypothetically bi-directional relationship which links marijuana consumption and school failure among students. To that end, we propose a simultaneous probability model, which is estimated by using the information provided by the three consecutive waves from the Spanish Surveys on Drug Use in the School Population [(1996, 1998 and 2000). Spanish Government's Delegation for the National Plan On Drugs, Madrid, Spain]. Our results confirm that whilst marijuana consumption is a determinant for school failure among Spanish students, we do not find evidence in the opposite direction. Moreover, explanatory variables, such as the presence of smokers at home, a mono-parental situation or unhealthy habits, among others, are good predictors for both marijuana consumption and school failure.