Riot And Their Ranking Systems

The system compare you with with everyone else playing the same champion in the same role and that's how you get the grade. So if you go something like janna mid and get 11/1/14 you'll have more chance to get an S than if you play zed with the same score. But the score isn't all that matter. It takes in consideration the total number of kills, your creep score, the tower destroyed and a few other things. So even if you have a nice score but with 0 cs (except if you're support) you probably won't get an S. It all depends on how the game goes.