The relationship between students’ subject-specific academic self-concept and their academic achievement is one of the most widely researched topics in educational psychology. A large body of this research has considered cross-lagged panel models (CLPMs), oftentimes synonymously referred to as reciprocal effects models (REMs), as a gold standard to investigate the causal relations between the two variables and has reported evidence for a reciprocal relationship between self-concept and achievement. However, more recent methodological research questioned the plausibility of assumptions that need to be satisfied in order to interpret results from traditional CLPMs causally. In this substantive-methodological synergy, we aimed at contrasting traditional and more recently developed methods to investigate reciprocal effects of students’ academic self-concept and achievement. Specifically, we compared results from CLPMs, from full forward CLPMs (FF-CLPMs), and from random intercept CLPMs (RI-CLPMs) with two weighting approaches developed to study causal effects of continuous treatment variables. To estimate these different models, we used rich longitudinal data of N = 3,757 students from lower secondary schools in Germany. Results from CLPMs, FF-CLPMs, and weighting methods support the reciprocal effects model, particularly when considering math self-concept and grades. In contrast, results from the RI-CLPMs were less consistent. Implications from our study for the interpretation of effects from the different models and methods as well as for school motivation theory are discussed.