In primary school, learning fractions is a central mathematical objective. However, the mastery of basic procedures involving fractions presents a difficulty for many students. The aim of the current intervention is to introduce structured cooperative learning as means to improve students' learning, particularly for average achievers. Previous research has underscored that heterogeneous groups might be deleterious for average achievers because they are excluded by the teacher learner relationships that is likely to take place between low and high achievers students. This intervention proposes structuring interactions in order to boost the learning of average achievers in heterogeneous groups. We hypothesize that highly structured cooperative learning should improve average achievers' understanding of the content-targeted in group work as well as progress in terms of fractions learning, when compared to low-structured cooperative learning. In this intervention, 108 fifth graders worked cooperatively in heterogeneous triads (a low, average, and high achiever). The triads had to express the length of one segment using three rulers with different sub-units and respecting three mathematical skills regarding fractions. Triads were randomly assigned to a low-structured or high-structured cooperative learning condition. In the low-structured condition, no specific structure was provided. (i.e., they organized their cooperative work as they wished). In the high-structured condition, each student became an expert for one part before working in the triad and endorsed different responsibilities. The results indicated that highly structured cooperative learning favors the understanding of the targeted task, especially for average-ability students. Moreover, students at all levels progressed from the baseline test to the post-test. Indeed, low and high achievers had the same progression in both conditions, whereas average achievers progressed more in the highly structured condition. Results are discussed in terms of new teaching methods that could efficiently increase average achievers' performances.