Aim: To investigate associations between participation-related constructs and participation frequency and involvement in inclusive schools. Method: In this cross-sectional study, teachers of children with additional support needs, including intellectual disability, autism, and learning difficulties, completed measures. Participation-related constructs were measured using the School Participation Questionnaire; participation frequency and involvement were measured using the Participation and Environment Measure for Children and Youth. A series of multilevel linear mixed-effects regression models with maximum likelihood estimates and bootstrap confidence intervals with p-values were obtained. Final models included participation-related constructs and participation, controlling for demographic and diagnostic confounders (including age, sex, language, level of school support, and autism). Results: Six hundred and eighty-eight children (448 [65.1%] males; mean age 8 years 7 months [range 4 years 10 months–12 years 13 months, standard deviation 2 years 1 months]) were assessed by 252 teachers. Across a series of models, participation-related constructs were consistently associated with more intensive participation (competence, environment, identity p < 0.001; symptoms p = 0.007), independent of confounders. More frequent participation remained associated with three of four participation-related constructs (competence, identity p < 0.001; environment p = 0.021). Age (p = 0.046), language (p = 0.002), and level of school support (p = 0.039) also remained significantly associated with frequency of participation. Interpretation: Children with additional support needs in inclusive schools may have several participation barriers. Policies and interventions to improve participation are needed.