The present study analyzed students' self-reports and teachers' assessments using the Japanese version of the Perception of Inclusion Questionnaire (PIQ), which measures students' emotional well-being, social inclusion, and academic self-concept. Data from 678 students (grades 5-9) and their 35 teachers were examined using classical test theory and item response theory. Categorical confirmatory factor analyses confirmed the three-dimensional structure of the PIQ for both the student and teacher versions. The measurement invariance was tested, and partial scalar invariance was established between primary and lower-secondary school students. A graded response model details item functionality across construct levels. A correlated trait-correlated method minus one (CT-C(M-1) model revealed that the consistency between the self-reports and teacher ratings was lower than the method-specificity of teacher ratings. The study underscores the value of using both classical test theory and item response theory for psychometric evaluations, thus proving the scale's reliability and validity in Japanese schools.