This grading system is a tad bit harsh.
It seems rather ludicrous to get an s with this current system. I just had two matches as Fiora where I thought that it'd be fitting I'd receive an S for how I played, with one match having me carry the team to victory after they had sadly died a few times to Katarina and Elise and the other match just being me playing very well and resulting in a swift victory. In the respective matches I went 16/6/6 with 184 cs at 33 minutes in and the other I went 17/4/7 with 177 cs at 32 minutes in. Both matches gave me an A+ for my efforts rather than the desired S. I don't quite know why I didn't receive an S for these matches as I placed many wards, was involved in most team fights and dealt a considerable amount of damage, and took down most of the towers. To me it feels like this system is very RNG in it's current state and that to receive an S is a roll of the die. Just felt like I needed to vent after losing out on getting my Mastery 7 baguette 