This study investigates the practices of public (high) school, private language institute, and public-private teachers. In particular, it aims at addressing the role of contextual factors, the variations teachers introduce to cope with them, and the degree of sustainable behaviour among these three groups of teachers. High school teachers consisted of those who taught only in high schools and the ones teaching both in high schools and private language institutes. For this purpose, classroom practices of 60 EFL teachers (N=20 per group) with 3 to 6 years of teaching experience and BA degree in TEF) were compared in terms of group/pair work, teacher talking time, L1 use, questioning, corrective feedback, and coverage of language skills. The findings of the study indicate that a significant difference exists among these three groups of teachers in terms of their practices. It is noteworthy that in the same teaching context of high school, the practices of teachers with and without private language teaching experience are significantly dissimilar except in the duration of pair/group work activities and the rates of repetition and explicit correction. This study suggests that high school EFL teachers with teaching experience in private language institutes subscribe more closely to the tenets of communicative language teaching and thus can act as powerful agents of sustainable language teaching in Iranian public schools.