It needs to be taught properly BEFORE they get to secondary school. My 7yo in Y2 uses punctuation & grammar correctly because she's taught it properly, as is my nipper in Y6. It's almost to late by the time they get into KS3 & certainly to late by KS4.
However, I see plenty, every day, who have been taught poorly & our English Dept spends 1000s of man hours teaching 16+ how to employ it correctly.