School engagement involves cognitive, emotional, and behavioral components that overlap conceptually. This conceptual ambiguity has led to measures that have either consisted of one general factor or separate correlated factors. However, neither approach can sufficiently account for both the uniqueness and the overlap of the subcomponents. The bifactor model has been recommended to determine the degree to which a measure is unidimensional versus multidimensional. In this study, we examined the validity of a multidimensional measure of school engagement in adolescence, the Behavioral-Emotional-Cognitive School Engagement Scale (BEC-SES; Li & Lerner, 2013), by comparing the model fit and predictive power of the widely-used one- and three-factor models with a bifactor model. Using data from 561 youth in Iceland (46% girls, M_age at Wave 1 = 14.3 years, SD = 0.3), only the multidimensional models (i.e., the three-factor and bifactor models) gave a good fit to the data. We then assessed the predictive power of the multidimensional models for academic achievement. The addition of academic achievement as an outcome variable to the bifactor model revealed that general school engagement, as well as specific behavioral engagement, predicted achievement. These findings are distinct from previous results using three-factor models, which indicated that behavioral engagement alone predicted later achievement. The results of the current study support the use of a bifactor model when using measures of school engagement.