Janna S?

1/0/18 and I got an S (although the only way I feel like I got it is because I went the total douche route and built a Mejai's because there's a similar pattern with all the supports and how you have to have a certain amount of damage dealt to champions...)

I wouldn't be surprised if 'support' Janna scores have been skewed by people playing hard AP Janna out of the bot lane, allowing them to rack up a lot more damage dealt, CS (later in the game), healing done, and probably even kills than a standard support Janna naturally would have. Since the system is measuring it based on a percentile of everybody playing the champion in that role, over time it'll start lending itself more and more to just those who are gaming the system.

It makes me sad I can't get a level 6 or 7 on my main because of this bull.

It is not difficult to get an S- on Janna. I think you need to get a couple of kills and have high kill participation. Damage to champs seems to have almost nothing to do with the grade. I usually do about 5-7k damage in a typical game. Getting a S or S+ seems more difficult after they introduced the level 6 & 7 masteries. I would suggest just trying to increase your kill participation and get more kills than deaths.