What is the marginal product of labor of these two teachers?

An experiment conducted in Tennessee found that the scores of second and third graders on standardized tests for reading and math were the same in small clrooms (13 to 17 students) as in regular clrooms (22 to 25 students). Suppose that there is a school that had 90 third graders taught by four teachers and that added two additional teachers to reduce the cl size. If the TN study can be generalized, what is the marginal product of labor of these two teachers?