Many models in science education have tried to clarify the causal relationships of affective variables on student performance, by presenting theoretical models, exploratory SEM (structural equation models), and confirmatory SEM. Based on the literature, the recent AS-TI-CU model scrutinised the most robust stimuli of conceptual understanding (CU): intrinsic attitude towards science (AS) and topic interest (TI). However, the confirmatory model has not been extended to estimate how students achieve in the secondary science or in Korea where student's disengagement in science is prevalent. Sampling 10th- and 11th-graders in Korea (N = 219), this study thus aims to clarify how the forth factor "school achievement (SA)" interacts in the structural equation modelling. The multiple-group SEM analysis in AMOS7 reveals that student's intrinsic AS stimulates their school achievement in both graders and that their topic interest abnormally discourages school achievement only in the 11th-graders. The findings provide explanations for the latent threats of negative attitudes and for the "age 14's dip". Lastly, how to form a theory of "persuasion of attitude change" is discussed for a further research question. (Contains 6 tables and 6 figures.)