Your specifications state:

Firstname Lastname list_of_grades(any number of grades)

Hence, you need to check to make sure a student has at least 1 grade before trying this division:

$students{$name_index}{avg_grade} /= scalar(@{ $students{$name_index}{
+grades} });
[download]

teachers are sneaky at times (or so experience would tell me). all else looks good though.

-enlil