While the relationship between self-determination and the academic achievement of students has been widely tested across cultures and groups, limited research has examined the antecedents of self-determination and their impacts on school engagement, which is the pre-requisite of academic achievement. The present study, based on the self-system processes model of motivation, tested the relationships between peer support, school support, self-determination, and school engagement in 118 secondary school students with special needs integrated into mainstream schools in Hong Kong. The results showed that school support has a significant indirect effect on school engagement via self-determination as a mediator. The correlations between school support, self-determination, and school engagement are all positive and significant, whereas peer support did not correlate with school engagement. Controlling for the factors of peer and school support, self-determination was the strongest predictor of school engagement for students with special needs. The current findings may highlight the importance of school support and self-determination interventions to enhance the school engagement of students with special needs in inclusive educational settings.