How can i get -s

Ok so i was playing 5v5 normal with real people not with bots.And my score was 25/2/2 and others were like 9/3 - 9/4 and i was leading most kills and like 2 deaths.And HOW i didnt get a -s and a guy irelia was 9/3/11 and he got a -s but me a better score didnt get a -s , Basically i need -s for mastery level 6 for zed.Any one knows how this works ? Thanks.{{champion:238}}
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