sub GPA_sort
{
     $value_a = $students->{$a};
     $value_b = $students->{$b};
     ($name_a, $gpa_a) = split(/\t/, $value_a);
     ($name_b, $gpa_b) = split(/\t/, $value_b);

     $gpa_a <=> $gpa_b
}