Over recent decades, educators have recognized the crucial importance of teaching environment and contextual factors in teaching and learning outcomes. Public and private schools are among those teaching contexts that have been in competition for teaching quality and effectiveness for many years. This study investigated the effectiveness of some English teachers in these two school contexts in a foreign language setting, Iran. Seventy-six public and private high schools in a small city in Iran were selected, and their English teachers' performance was evaluated by an external observer and self-evaluation using a high-inference observation instrument and a questionnaire. The result of the data analysis showed that English teachers who worked in private schools were more effective teachers than their colleagues in public schools. The results also revealed that teacher effectiveness and their years of teaching experience and age were weakly but significantly related. Furthermore, it was found that teachers' type of university degree and the location of schools (disadvantaged vs. privileged areas of the city) were not related to teachers' effectiveness.