<p>It is important to look more closely at how generative AI tools like ChatGPT affect student writing as they become more common in the classroom. At a military boarding high school in Vietnam where students are not allowed to use personal electronic devices outside of class, this study examines the effects of ChatGPT on argumentative writing among intermediate-level EFL learners (CEFR B1–B2). A quasi-experimental design with little outside interference was made possible by this rigorous environment. 42 students, ages 16 to 17, were divided into two groups over the course of six weeks. While the control group received in-person teacher instruction, the experimental group supported writing tasks using ChatGPT in a supervised computer lab. Weekly writing essays were written by both groups, and their performance was evaluated using a rubric for pre- and post-tests. Semi-structured interviews, classroom observations, and reflection journals were used to collect qualitative data. The experimental group initially outperformed the control group in fluency and structure, according to the results; however, formulaic writing and a decreased level of critical engagement were indicators of an over-reliance on AI. In contrast, the control group gradually improved its originality, argumentation, and cognitive ownership despite starting out slower. Additionally, students in this group reported feeling more confident and growing personally. These results imply that, even though ChatGPT can improve writing at the surface level, it might impede deeper learning if instructional scaffolding isn’t used. For technology to complement critical thinking and learner autonomy rather than replace them, it must be carefully incorporated into pedagogical frameworks.</p>