The aim of grading at schools in fact is to make a sequence of student from the best to the worst with distinction who has done his work satisfactory, who should change the school to more or less demanding etc. The grades are kind of quantitative information expressed by numbers let’s say 1-5 (1 is the best).
But if we study the problem in details, we find that even such number includes subjective elements.
Let’s take only one subject – English language for example. There might be more objective criteria as:
Spelling
Grammar
Word supply

and less objective criteria as:
Volubility
Accuracy of Expression
Words matching situation

Each of above criteria are evaluated by teacher (some tests can be evaluated by machine but alive person compose the test and set evaluation rules) and always includes some of his/her individual point of view. The final grade is usually not made by some mathematical method but includes also evaluation of personal qualities as diligence, aspiration, behaviour, discipline and others.

Description of situation everybody knows should illustrate that numerical data are not necessarily free of personal influences, mistakes and half-truth. None the less they are usually taken as more reliable and more correct.