i've done so much worse and gotten S's in the past, but i've gotten nothing but A's from all these sorts of games
honestly there has to be something currently wrong with the game that needs fixing, like Meddler was tuning the rating system and accidentally changed "300 cs" to "3000 cs" when no one was looking
Paquay2 votesAs a support main, I think the game is telling you to buy more control wards. ;)
Ohikno1 votesProb just that the average gameplay got better with the champion you are using. Who are you using and getting A-'s with?
Ok sure but why1 votesVayne top, Darius top, Veigar mid, Blitz mid, and Lee jg
all had 8:1 kda or better and 5+ turrets, but riot don't care about that
Ohikno1 votesStraaaaaaaange.......
Torgaz1 votescs/min is huge for your grade unless you are playing support. you want an S you have to get high enough cs compared to others playing that champ
Edit: also look at things like objective contribution and wards placed. these matter more to a support but they will also help
Dynikus1 votes129 cs in 30 minutes. I question that you got an a- for that though. Just CS more and you'll probably get an S. What did you get when you went 23/6/15 with 6 cs/m as veigar?
Ok sure but why1 votesTbh I got an S with that one, because I went from 5/6 to 26/6
Dynikus1 votesor because you had decent cs/m as well as k/d/a.
Ok sure but why1 votesbecause i trolled my lane and took their cs with every gank
Dynikus1 votesHave you considered working on your farming so you can get good cs without doing that? CS is important for a lot more than just your grade. 1 kill is about equal to 16 CS. If you're up 3 waves on your enemy laner but are down a kill, you still have the gold lead.
PrinceOfStorms1 votesRating is based on how people do on average with the champion in your assigned role. So even champs like Janna can get an S based on how well they did.
Ok sure but why1 voteshow many people go 15/2/13 as vayne top
Valderis Vandala1 votesRiot measures what they call "core metrics" so that, in their words, "so that if you're doing the right thing for your team, we're going to score you well for it." http://boards.na.leagueoflegends.com/en/c/gameplay-balance/bZL8vAWw-champion-mastery-qa?comment=00540000
You get compared to other people in your region who played the same champion as you in the same role as you, the higher your score amongst these people the better your grade will be. https://support.riotgames.com/hc/en-us/articles/204211284-Champion-Mastery-Guide
Also, there are a lot of people giving false advice, a lot of people believe that kda and cs is somehow a direct factor in your grade. These people are most likely wrong as it has been observed that a player can have a pretty terrible kda and cs and lose the game yet still achieve a S level grade. http://boards.na.leagueoflegends.com/en/c/gameplay-balance/cTJF3oUc-s-on-feeding-trundle
Riot has also noted that even the best of players cannot consistently get the highest grades (though they do get them more often), which leads me to believe there is at least some degree of luck involved.
The true workings of the system are unlikely to ever be revealed to us, so the best you can do is just doing your best and not worry about it. Good luck.
Dynikus2 votesin that same thread http://prntscr.com/e2svjd
It says the 
that went 0/3/17 got an S on 
after someone said something in chat.
I'd be willing to wager that's some photoshop, my friend.
You seem to have this strange vendetta against anyone that says CS matters for your grade, but there hasn't really been anything to say otherwise.
Valderis Vandala1 votesThank you for bringing that to my attention.
shadowlord7231 votesDespite the KDA playing an important part in getting you an S- or above, I can say that MANY other factors determine the grades you get.
Your grade depends on how other players perform while playing your selected champion since the beginning of the season. It shows how well you do compared to those players. You get an S if you performed way better than the average people playing that champion.
Try picking champions with lower play rates 
or champions which are very recently released. The fewer people that play that champion, the more likely you are able to place yourself as better than them (unless those very few players are gods at that champion). As for champions recently released, I was referring to our most recent reworked 
when not many people knew how to play him. With that in mind, you can easily become better than other players the more you play the new champion, allowing you to achieve an S. A bit too late to try that on the new Warwick at this point. Just wait for a new champion release.
Depending on the champion, the KDA will matter. Champions like 
don't really focus too much on the KDA (trust me, I'm one of the few mains out there). Due to certain champions being able to benefit from dying, it is technically alright to have a bit more deaths than other champions. Your KDA still needs to be good though. With these champions, you can go 17/8/12 and still get a possible S. There are also other champions which focus more on CS rather than KDA, with 
being a prime example. Still gotta maintain a fine KDA though, but at least not a godly one. But try to get as much CS as you really can with those champions and you'll be fine.
CS is just as important as KDA when getting an S. Not only does the number of CS matter, but your rate of the CS you obtain matters as well. It is ideal to get around 100 CS by the 12-15 minute mark and 200 by the 25 minute mark. A lot of people who complain about not getting an S tend to overlook this aspect. Stealing the enemy jungler's CS can also aid with reaching that S grade when it comes to CSing. Doesn't help too much, but still useful for the grading system to consider.
You must have great map presence to help get an S. Staying in your lane throughout the whole game isn't really helping out the team much. You must roam to other lanes and assist them with pushing and/or killing the enemies Choosing champions that specialize with roaming 
can aid you with how much presence you give out throughout the battlefield.
Junglers also specialize with this as it is ideal for them to constantly gank. Since they don't really have a lane and are often roaming around the map, they can get off ganks wherever they possibly can.
Getting objectives is definitely important. These include getting Dragon, Rift Herald, Baron, and (surprise surprise) towers. When you or your team aim for these, try to deal as much damage to them as possible or last hit them. When you last hit these objectives (or deal enough damage to them), the game will count it as an objective you have taken and will use it to help determine your grade. When getting towers with a minion wave with you, try to last hit the towers so that the game will record that you have taken an objective. If the minions last hit it, the game won't record it. That's usually why players try to last hit the turret.
Your early game performance also helps with improving that grade of yours. Not only must you get as much CS as possible, try to get the enemy's tower first before they get yours. Same thing applies with First Blood. You are better off having your team get First Blood rather than giving away First Blood. In short for early game: get CS, kill enemy laner first, and destroy their tower first. Oh, and while you're at it, don't die too much from enemy ganks.
Try to place down wards as much as possible. This helps out with map control allowing for your team to gain better vision of the area. Control Wards are much better as they can also reveal stealthed units and remain on the battlefield until destroyed. The more wards you place, the better.
When it comes to support, what matters most is your KDA as well as how much you help your team. Make sure that while in a fight, your team survives and the enemy dies. Later on in the game, roam around the battlefield and assist your other teammates in getting kills and objectives. Don't just help out your ADC, HELP EVERYONE EVERYWHERE!!!
Your grade is also determined whether you win or lose. If you lose, the system will determine how well you played to try and achieve victory, how well you did to defend your nexus, as well as how well you tried to help your allies despite falling behind. If you are good enough, you'll end up with an S- despite losing. However, if you were to win yet perform the exact same way as mentioned, you'll most likely end up with an A.
Play the upcoming ARURF. Lots. No, really, play it. Now only is it fun, but it will give you a better chance to achieve an S grade. Due to the 80% CDR as well as the manaless abilities, you are almost guaranteed to perform a lot better compared to average players who play the champion you selected. Last I checked, the grading system doesn't separate your performance in URF from your performance in a normal game. Pretty much an easy S. Only downside is that you won't be able to obtain Mastery Tokens.
These are my tips for achieving an S grade in case someone needs it. Hopefully I didn't leave anything out.
Quepha1 voteswhat champ? what role? damage done? damage done relative to team? damage taken relative to team? cs? objective participation? length of game? how well were you doing early/mid/late?