At school, pupils often cooperate on common projects and must coordinate their different individual actions. However, grades are pervasively used even in cooperative situations, which make the pupils' differences in achievement and their relative rank salient and may reduce their inclination to work constructively with others. Thus, we hypothesized that grades would elicit disruptive interactions and reduce performance in a cooperative cognitive-motor task necessitating inter-individual coordination of members. In a study with 5th graders, grades (vs. a neutral concept) were primed at the onset of a cooperative group interaction. Results showed that, although pupils were set to work cooperatively, priming grades (vs. neutral concepts) harmed inter-individual coordination by eliciting more negative dominant behaviours among pupils during interactions, which decreased group performance.