Score Rank

I still can't get my zac mastery,,, Every game is an A+ either I farm jg for 30 mins and win or carry a team via ganking alot either way this is starting to get frustrating. I'm a scrub at this game ill admit it but still when I have games like this one and still cant get an S idk what to do... btw both yas and zed got s+ how does this system fking work????