I'm assuming by "exam" you mean a "gateway/quiz" type assignment. You can get the usual statistics for the scores on the test by using the Statistics link on the left sidebar of the WeBWorK page. This includes data on the distribution of scores on the assignment (test), including the median. That said, I don't think the average shows up explicitly anywhere. For that type of data I usually download a CSV file of the scores for the class (using the Student Progress page) and use a spreadsheet.
their performance compared with the rest of the class. It is a nice way
to reveal details without revealing private info about any student.
It would be nice of Webwork could generate this automatically for the class but if not I understand and can easily get the CSV to do it myself.