In a school, a group of senior students and junior students are organizing a charity event. Some senior students are assigned to work with some junior students as co-leaders. No junior student is assigned more than one senior co-leader. If $\frac{2}{5}$ of all the senior students are paired with $\frac{3}{7}$ of all the junior students, what fraction of the total number of senior and junior students have a co-leader role?