My son is a Graduate Assistant in the English Deparment at OU and teaches Comp I and II. He gave a "C" on one of the papers and the student came in his office furious because he had been in English Honors in High School and no teacher had given him a "C" before on a paper. He demanded to meet with the professor that is the mentor to several of the grad students. My son took the student and his paper to the professor who reviewed the paper, looked at the student, and told him he thinks my son could have given him a "D" and still been right. He then proceeded to tell the student that the corrections my son had made were proper.
The professor gave the student a book on English Grammar and told him to read the Rules, print out his paper again, make the corrections, and then tell my son that he was still wrong. The student came in and apologized the next day to my son and told him that none of his English Honors teacher's in High School had bothered to correct his work, just gave him an "A." He also asked him to please print out the English Grammar tips my son had handed out the first day of class he didn't think he needed and trashed.
Been interesting to hear my son talk compared to when he was a student -- totally different perspective and he tells me if he ever forgets what he was like as a student to remind him.
You raised a fine son there - God bless! :-)