This study examined the associations between teachers’ beliefs and behaviors related to inclusive education (IE) and their evaluations of social and academic competences in students with Autism Spectrum Disorder (ASD) – important indicators of IE. Utilizing the Theory of Planned Behavior (TPB), the research investigated how teachers’ intentions to implement IE and their supportive practices affected the social and academic competences of students with ASD. The study involved 267 teachers from 78 mainstream secondary schools in Hong Kong. Teachers completed a questionnaire assessing attitudes, subjective norms, perceived behavioral control, and intention to implement IE. Information on teachers’ practices supporting students with ASD and their evaluations of social and academic competences of these students was also collected. Structural equation modeling (SEM) was used to analyze the relationships between these variables. Correlation analysis revealed positive and significant associations between teachers’ beliefs, inclusive practices, and their evaluations of social and academic competences of students with ASD. SEM indicated that perceived behavioral control significantly and positively influenced the social and academic competences of students with ASD through teachers’ inclusive practices. This study highlights the vital role of teachers’ control beliefs and inclusive practices in fostering the social and academic competences of students with ASD. By applying the TPB and SEM, the research contributes to understanding how teachers’ intentions and behaviors in implementing IE impact students’ outcomes. The findings offer practical implications for enhancing inclusive education practices for supporting students with ASD in mainstream schools. Copyright © 2024 The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.