A colleague and I were talking about a student whom we shared when I remarked, that he is a B student. The colleague, who was at the beginning of his teaching career, asked me how I knew this. This is what I told him.
Say a student comes to your office hours asking for help with a homework problem. During the course of the help session you scribble a few drawings and an equation or two on a piece of paper. Watch the student's behavior.
The A student will say, "Oh, how silly of me not to have seen seen this. I feel embarrassed."
The B student will say, "Can I have that piece of paper?"
The C student will leave without saying anything but will come back an hour later and say, "How was that again?"
The D student will not come to office hours.
The F student will not do homework problems.
I mention this to indicate that after a few years of teaching, one gets to develop a sense for evaluating independently of test scores where the student stands on the scale Excellent, Good, Fair, Poor, Failure. One doesn't even need to meet the student face to face to rank him/her on this scale. Just look at the reactions of OPs to requesting homework help, count the number of posts needed to get to a resolution and examine the last post by the OP. Here is a suggested scale and please correct me if I'm wrong.
The A student will post twice, saying in the second post "Oh, how silly of me not to have seen seen this. I feel embarrassed. Sorry for wasting your time."
The B student will post a handful in which he/she provides constructive answers to the leading question "what do you think you should do next?" and in the last post says, "I can finish this on my own now. Thanks."
The C student will require a dozen or more posts and often needs to have the question "what do you think you should do next?" broken down into sub-questions and prods of the "Note that ..." variety. Nevertheless, the OP sticks around until the question is answered. The last post is something like "Thank you very much for your patience, I learned a lot."
The D student quits after a handful of posts with the question left unanswered.
The F student posts once and then disappears even though several posts with helpful suggestions from different perspectives have been posted.
Test scores are only one dimension contributing to a student's standing on the Excellent, Good, Fair, Poor or Failure scale. Native intelligence and attitude are additional dimensions, just as important, if one is to encapsulate a student's quality into a single letter grade. How to fold these dimensions equitably into the overall assessment of a student has been of some concern to me during my teaching career. My resolution of the issue was not the same as that of my colleagues at my institution and certainly not the same as other U.S. or Canadian institutions. Therefore, I agree with others on this thread that it is meaningless to attempt letter grade comparisons.