**These are just my theorys but it works all the time for me**
Take objectives when possible:
* Call baron (without failing/throwing)
* Call dragon (without failing/throwing)
* Call turrets/inhibitors (without failing/throwing)
When you make succesful calls, the chance getting S+ in grade is very, very high
Don't go for kills when you can take objectives
Don't go for creeps when you can take objectives
Go for kills/creeps when you can't go for objectives
Be involved in teamfights
Don't join the teamfight if you think you have better **successful** decision makings that leads your team into victory.
**Summary:** you need to have VERY GOOD decision makings, and KEEP HAVING very good decision makings until the end of the game! Decision makings that leads you into **victory**.
So, when you have the best KDA, and you get A+ but your support get S, then it pretty much means that your support has pushed your team into victory **better than you**. Hey... it's a support ;)
Yet, A+ is actually a good grade!
Hope this all helps!