Riot, please, fix your grading system.
Now, before we get into the actual problem, I'll say here. You HAD good intentions. The grading system was a great way to have everyone try their best to get that S to get Mastery 7, and it was working. For a little while.
We don't know how to get S' and you said you will never reveal how to do so, and everyone is trying to find the way, but the problem is the grading system itself. Its' system works like this.
Everyone is trying to get their Mastery 6 & 7 tokens to level up their favorite champions. The way we will determine what is the best performance, we compare you with that of the best players in that champion and lane.
So, everyone is technically competing to get those high scores to get those 6 & 7 tokens, but everyone is performing too similarly, making it hard to get those tokens. For example, a Riven goes 12/3/8 by the end of the match, and she ends up getting a A+. If she were to get a 18/2/4, she would get a S-. Because, everyone, at this point, is trying their absolute best to win the lane to get those tokens. Heck, I bet that even some people smurf to easily get them.
The point of this post is to ask Riot to PLEASE change the grading system. Everyone is performing too similarly and just making it impossible to get S'. Some people at this point, put their champion in a lane or jungle in which the champion does not work to get that S. I've seen a Vi bot lane losing to the enemy Ezreal and Janna, saying "I want to get that S so badly." So, people at this point, are trolling to get that S. It's a shame it couldn't have been done differently, because now, people trying to get their S, fair and square, don't get it at all, and if they do, that's the only one they'll get in a long time.