The median mark is the middle mark when the marks are arranged in ascending order. First, let's arrange the marks in ascending order: 5, 6, 7, 7, 7, 8, 8, 9, 9
Since there are 9 students, the median mark is the average of the 4th and 5th marks. The 4th mark is 7 and the 5th mark is also 7, so the median mark is 7.