9. Four schools each send 3 representatives to form $n$ groups to participate in social practice activities (each representative can participate in several groups). The rules are: (1) representatives from the same school are not in the same group; (2) any two representatives from different schools are in exactly one group together. Then the minimum value of $n$ is $\qquad$
9.9 Group. First consider the representatives of cities A and B, let the representatives of city A be $A_{1}, A_{2}, A_{3}$, and the representatives of city B be $B_{1}, B_{2}, B_{3}$. They can form 9 pairs: $A_{1} B_{1}, A_{1} B_{2}, A_{1} B_{3}, A_{2} B_{1}, A_{2} B_{2}, A_{2} B_{3}, A_{3} B_{1}, A_{3} B_{2}, A_{3} B_{3}$. According to the problem, the number of groups $k$ is no less than 9. To minimize the number of groups, the number of people in each group should be as large as possible (up to 4 people), the following nine groups meet the requirements: $\left(A_{1}, B_{1}, C_{1}, D_{1}\right),\left(A_{1}, B_{2}, C_{2}, D_{2}\right),\left(A_{1}, B_{3}, C_{3}, D_{3}\right),\left(A_{2}, B_{1}, C_{2}, D_{3}\right),\left(A_{2}, B_{2}, C_{3}, D_{1}\right),\left(A_{2}, B_{3}, C_{1}, D_{2}\right),\left(A_{3}, B_{1}, C_{3}, D_{2}\right),\left(A_{3}, B_{2}, C_{1}, D_{3}\right),\left(A_{3}, B_{3}, C_{2}, D_{1}\right)$. $\left(B_{1}, C_{3}, D_{2}\right),\left(A_{3}, B_{2}, C_{1}, D_{3}\right),\left(A_{3}, B_{3}, C_{2}, D_{1}\right)$