This study aimed to investigate the perceptions, interactions and behaviours of different-ability college students when they worked on different types of assessments. Two classes of 145 Vietnamese college students participated in this three-month study. The students were assigned to mixed-ability groups, each of which consisted of five students. The results show that assessment designed as a group project helped close the gap in communication and interactions between different-ability students within groups. When the students engaged with assessment as a group project, all levels of performers increased productive learning behaviours and provided more relevant verbal help and assistance to each other. Importantly, this type of assessment created various opportunities for the low performers to participate in, and make a contribution to, group tasks. As a result, all group members became interested in working with others. They perceived cooperative learning groups as being enjoyable and fun.