exams do not refelect the actual level of a child, there are many factors thay may interfere the day of the exam and thus restrict the child's abilities. So a bad grade does not necessarily mean that a child is not a good student. Also, i think that grading should not be made in numerical characters, especially when it comes to children. They may feel offended or lose self confidence if their mates get better marks.
I think that remarks are enough.