The academic achievement of diverse kindergarten to grade 12 students in inclusive classrooms implementing the Three-Block Model (TBM) of universal design for learning was investigated. Fifty-one teachers and 684 of their students, including students with disabilities, students who were English language learners (ELL), and Indigenous students, participated in treatment and control groups. Results indicated significant differences between the two groups on measures of academic achievement, as reflected in a rubric that assessed levels of critical thinking. This was the case for typically developing students as well as those in the three subgroups, with large effect sizes in all cases. These findings suggest that the combination of social and emotional programming with universally designed instruction holds promise for improving the academic achievement of a broad spectrum of learners. Results are discussed in regards to implementation and training needs, and recommendations are made for future research.