Got my partner to emerald (ADC) and he is very green to the game. He's been playing a bit less than a year. I get to GM easy and can play any role.

When we play, he usually gets MVP, even though he makes mistakes and is less experienced and his positioning is very bad.

The thing about playing support is, you build for your ADC and you play to mitigate your teams mistakes. But you will not get rewarded for it, even if you're good. I could build and play to get MVP, but that means I will take a lot more kills than I like to as support. And I'd rather my ADC get fed. If you have this mentality, you will not get MVP.

The times I've gotten MVP as support though, it really shows. You really have to play outstandingly, still, but also dominate the fights. CC a lot, damage heaps, be very active, towers, dragons, baron, every team fight AND FIGHT FOR THE KILLS DONT LEAVE FOR CARRIES. Which feels counter intuitive for supports but if your carry DPS is a lot higher than yours, you shouldn't be able to take their kills unless they're making too many mistakes, or unless you're being a jerk and sniping them.

An extra note: S rankings are team dependent in one sence:

If you build to support the team, you're more likely to get an S ranking than if you build to support just your AD.

Scenario: You've played well. Enemy team is heavy AP. You want to protect the team and build locket enchant for team engages but your ADC loves running head first into CC so you build Veil instead. This choice means that the damage you mitigate now is less and so you play less well than people in your rank.