Are objectives also considered in your grade?
solo'd two drakes, got 4 towers but ended with a 2/4/1-ish score to get a B+.
From seeing posted screenshots of people getting an S rank and playing with people that get S ranks, the biggest thing with an S rank seems to be your performance compared to your Rival within any particular game. If you go 5/0/12 with 100 CS as a jungler but the enemy jungler goes 5/0/12 with 100 CS: you're not getting an S. If you go 5/2/12 with 150 CS and the enemy jungler goes 0/12/5 with 50 CS: you might get an S.
Concerning objectives: it's probably a similar scenario. End number doesn't matter so much as the difference.
Though in this case: objectives are probably considered on the team level over individual level. As-is it doesn't matter who lands the killing blow on objectives, only thing that matters is they die. Also: maintaining the map-pressure necessary to allow a jungler to do something like solo Dragon is still more of a team thing than an individual thing, so objectives buffing your score is probably more your team vs. there team.