Gender contents, textual or visual, in instructional materials can be considered as a hidden curriculum, and may either hinder or advance equality. How teachers interpret and respond to this hidden curriculum in their classroom discourse, however, can play a crucial role in creating positive change. This research seeks to understand how teachers work with gender-sensitive contents in a newly-published English textbooks series for lower secondary education in Vietnam. To what extent are teachers aware of genderedness in textbooks, and how do they respond to it? Approaching gender as a social construct, using multi-dimensional methods, the study critically explores teachers’ experiences with the textbooks – their classroom practices and underlying perceptions – through 18 classroom observations and 12 follow-up interviews with 12 teachers in four lower secondary schools. Results indicate that neither the hidden curriculum of gender was recognized nor a critical mediation of it was practised. Gender content, whether ‘traditional’ or ‘progressive’, was largely absent from classroom discourse, as teachers prioritized transferring linguistic knowledge. Little space was allocated for promoting students’ critical thinking and self-reflections, including those about gender-related content. Teachers did not often pay full attention to different nuances and dimensions of gender issues, while sometimes their teaching design and interaction were found to be affected by their own bias – consciously and unconsciously. The study reveals that although teachers were later willing to reconstruct their perceptions and future practices, and were able to concretize their pedagogical intentions, enacting teacher agency in relation to critical pedagogy and gender equality in the contemporary English as a foreign language (EFL) classroom in Vietnam might remain a challenge. Based on the study’s findings, pedagogical implications that may enable teachers to be potential agents of change were suggested.