Receiving report card grades is psychologically salient to most students and can elicit a range of affective reactions. A 3-wave longitudinal study examined how grades shape students' (N = 375; M age at Wave 1 = 12.6 years) school engagement through the affective reactions they elicit. Emotional and behavioral engagement were measured at the start of secondary school and 6 months later. Halfway through this period, students' positive and negative affective reactions to their 1st report card in secondary school were assessed. As expected, lower report card grades predicted lower emotional and behavioral engagement in spring, when controlling for prior levels of engagement. These links were mediated by students' affective reactions. Boys and children who perceived the performance norms in their class to be high were more affectively reactive to their grades, which resulted in a stronger indirect effect of grades via negative affect on emotional engagement. Complementing the traditional view that grades are "consequences" of school engagement, the current findings suggest that grades function also as antecedents of school engagement.