Grading system

As I said on another post, I usually play support and I really I'm really upset about not getting an S. Like I can have the perfect game or even carry the game with my champion abilities and still getting a B or A. Like what do I have to do to get an S? At least can riot give us an explanation about what are the "factors" of achieving a better grade in our matches? I've read that on 6.13 they are going to fix supports but still a board with what are you judged on getting S would be really good. I've heard that you are judged from the average players but how should I know the average player's score?