For whatever reason, Rito put huge emphasis on CS for grades for jungler even though not all junglers CS the same.
I find that you can get an S on almost any champ just by doing these things:
-Have a positive KDA (you can even go like 3/2/8).
-Last hit turrets, try to get at least 3 turret take downs.
-get a good vision score (drop at least 2 control wards and you are probably good)
-FARM, FARM, FARM
You can pretty just ignore your team and just AFK farm and if you get over 250 cs on a jungler it is almost always an S for me.
I went like 10/1/8 and carried my team to a win on Skarner and just because my CS was only like 150 I only got an A. I did a game where I went 2/1/10, but had like 280 CS (my kill part wasn't even very good) and I got an S. I went 16/1/14 on Karthus and got an A+, I am like wtf? What do you have to do?
Unfortunately, grades do not always match that you were a good teammate. Riot needs to redo the weights, or reconsider how they grade people.