a 12/12. Two people grade your writing test. One person grades it out of 6 and the other person grades it out of 6. Then the two scores are added. So if one person grades it a 4/6 and another grades it as a 5/6 you get a 9/12.
If the people who grade your essay have a difference of 2 or more in their scoring than a 3rd person comes and scores your essay too. So if one grades you as a 3/6 and another grades you a 5/6 would require a 3rd grader.