How to get S(+) grades as a jungler ?

And I really really don't have a plan how the system works. In over 100 jungler games I got the S garde only 1 singe time. I played Jungle Rengar, was 7/0/2 and the enemies surrendered at 26 minutes. We had 1 drake. {{champion:107}} {{champion:64}} I really struggle at getting them lvl 7 {{champion:268}} {{champion:41}} But them I got lvl 7 within 3 days. Tell me if u know how to get an S as a jungler, pls. ^^
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