During Student-Teacher Negotiations at a Certain High School,teachers Seek to Change

During student-teacher negotiations at a certain high school,teachers seek to change the number of times students are allowed to be tardy,saying that the semester "average" is 8 days of tardiness per student.The students counter that the "average" student is only tardy 4 times per semester.Explain how both sides may be correct,identifying the measure of centre you think each side is using and why the difference might exist.