Children who use augmentative and alternative communication (AAC) are multimodal communicators. However, in classroom interactions involving children and staff, achieving mutual understanding and accomplishing task-oriented goals by attending to the child's unaided AAC can be challenging. This study draws on excerpts of video recordings of interactions in a classroom for 6-9-year-old children who used AAC to explore how three child participants used the range of multimodal resources available to them - vocal, movement-based, and gestural, technological, temporal - to shape (and to some degree, co-control) classroom interactions. Our research was concerned with examining achievements and problems in establishing a sense of common ground and the realization of child agency. Through detailed multimodal analysis, this paper renders visible different types of practices rejecting a request for clarification, drawing new parties into a conversation, disrupting whole-class teacher talk-through which the children in the study voiced themselves in persuasive ways. It concludes by suggesting that multimodal accounts paint a more nuanced picture of children's resourcefulness and conversational asymmetry that highlights children's agency amidst material, semiotic, and institutional constraints.