As a teacher and DA, I'd like to add an observation to the following:
Let's see how the grades varied against the average. This is called computing the variance.
A very large variance means that the students' grades were all over the place, while a small variance (relatively close to the average) means that the majority of students did fairly well.
The interpretation of variance is dependant on the contextual variables that surround the subject. In the context above, a large variance in student grades is often attributed to either a diverse array of zones of proximal development amoungst the students, lack of comprehension regarding the subject material, or other external factors. A small variance in this context often indicates that students are at a similar level of proximal development, not necessarily that they "did fairly well". For example, a test could yeild a mark average with a small variance, but that average could be 40 marks out of 100.