Purpose: This study evaluated the efficacy of a collaborative intervention where a speech-language pathologist (SLP) trained mainstream secondary school teachers to make modifications to their oral and written instructional language. The trained teachers' uptake of techniques in their whole-class teaching practices and the impact this had on the language abilities of students with language impairment (LI) were evaluated. Method: Two secondary schools were randomly assigned to either a trained or a control condition. A cohort of 13 teachers (7 trained and 6 control) and 43 Year 8 students with LI (21 trained and 22 control) were tested at pre, post, and follow-up times--teachers by structured interview and students by standardized spoken and written language assessments. Results: Significantly increased use of the language modification techniques by the trained teachers was observed when compared to the control group of untrained teachers, with this increased use maintained over time. Results from the trained group of students showed a significant improvement in written expression and listening comprehension relative to the control group of students. Conclusion: This randomized controlled trial is one of the first investigations to evaluate a collaborative intervention that links changes in mainstream secondary teachers' instructional language practices with improvements in the language abilities of adolescents with LI.