In calculating the average class mark, teachers are used to sum up the marks of all student and then divide the total marks by the total number of students. It is well known that the artifact of this traditional method is the sensitivity of extreme marks. Even a very small proportion of students get fail or extremely outstanding in the subject, the average class mark will be influenced to a lower or higher value. Thus, the real standard of the class may not be accurately reflected. In order to reduce the sensitivity of the extreme marks to the average class mark, a statistical approach, which could down-weight the marks of the extremely outstanding or poor students, is proposed in this paper.