This mixed-methods study attempts to explore the effects of TBLT on the development of Iranian high school students' writing skills. To this end, two entire classes were selected in Shahed high school in Borujerd City, Iran. The groups went through pre-test, intervention, post-test, and delayed post-test procedures. In addition, using a classroom observation checklist, the researchers checked how much of the TBLT tenets can be implemented in real classes, and a focus group interview was used to further investigate the students' perceptions about the effectiveness of TBLT. The collected data were analyzed through repeated measures: ANOVA, a calculation of percentage, and content analysis. Findings showed a statistically significant difference between the experimental group and control group in relationship to their writing skills. Moreover, the results of the classroom observation checklist disclosed that the principles and procedures of TBLT were implementable in Iranian high school classes. Furthermore, the findings of the focus group interview revealed that the participants have positive attitudes toward TBLT. Finally, a range of implications is suggested for the different stakeholders.