Background: Although language and communication difficulties are common in secondary school students, there has been limited research into the efficacy of interventions for adolescents with language and communication difficulties. Aims To investigate the efficacy of teaching assistant (TA)-delivered narrative and vocabulary interventions to mainstream secondary school-aged students with language disorder. Methods & Procedures: A randomized controlled trial (RCT) of a language and communication intervention was used to evaluate the efficacy of vocabulary and narrative interventions to improve the vocabulary and narrative performance of adolescents (mean age = 12.8 years) with language disorder. The language and communication programmes (narrative, vocabulary and combined narrative and vocabulary) were delivered by TAs in the classroom, three times per week, for 45?60 min each, over 6 weeks, totalling 18 sessions. Standardized and intervention-specific measures were used as outcomes. Outcomes & Results: Twenty-one schools with 358 eligible participants were recruited. The three intervention groups showed significant improvements (d = .296) on a narrative latent variable defined by a standardized narrative assessment (the Expression, Reception and Recall of Narrative Instrument?ERRNI), but there were no significant improvements on an overall vocabulary latent variable compared with the waiting control group. Differential effects were found on some non-standardized intervention-specific measures with the narrative group making significantly more progress on narrative tasks compared with the waiting control group, the vocabulary group showing the same pattern on specific vocabulary tasks, and the combined narrative and vocabulary group making significantly more progress on some of the intervention-specific narrative, and all the intervention-specific vocabulary outcomes compared with the waiting control group. Conclusions & Implications: It is possible to improve narrative but not vocabulary skills, as assessed by standardized measures, in secondary school students with a relatively brief group TA-delivered intervention. There were differential effects for both narrative and vocabulary with intervention-specific measures. Future work is required to explore whether more intensive and longer lasting interventions would be more effective and to identify which students in this age group are most likely to benefit from such interventions.