Grading Scale for the Support Role

I am lost on how the support role in League is graded. Is it on the same scale as every other role or is there a grade for an individual role based on Creep score, kills, deaths, and assists. I ask because I have had several games that I thought should be an easy S, but instead, I got either an A, A- or an A+. If the grading of the support role is that same as the other four roles that would make much sense. Seeing as how I am to support and dependant on kills, Kills should be a factor in grading. As well as CS, the only CS I get is from Relic shield upgraded which only has 3 stacks. Which isn't a lot of a 30-40 minute game. What is the requirement for me my self as a support to getting an S? Is it dependant on the assists, wards placed/destroyed? I have no idea what it is which is driving me crazy. All I want are boxes and get masteries for champs to show growth.