Given the complexity of dialogic argumentative writing and the requisite instruction needed to support student writers, we describe the instructional practices of an English language arts teacher and her culturally and linguistically diverse classroom of 10th graders' writing during key moments in two instructional units during school year 2022-2023. Adopting microethnographic discourse methods to study key events, our findings demonstrate that the teacher used "dialogic opportunities" to provide contextualized support for her students' understandings of essay assignments, relevant topical knowledge, acquisition of argumentative genres, and strategic practices for composing. Our year-long collaboration with the teacher and her students suggests that while English language arts teachers need to have a deep understanding of how to teach dialogic argumentative writing, they also need to reconsider the larger curricular contexts for such writing and how instructional choices shape these contexts.