The average is, in fact, a number that represents a group of numbers. It is the average, its center, therefore, it represents them.
When we ask, for example, what is the average height of the third grade B students, in reality, we are asking what is the height that would represent all of them.
It is true that each student has a different height, but the average collects the median measure of all the heights and results in a representative number of all of them.
The more short children there are in the grade the lower the average height will be, the more tall children there are in the grade it will be higher.