This is what I come out with:
Let y\in<S>^{\bot}, then <y,w>=0, for all w\in<S>.
But S\subseteq<S>.
Hence for y\in<S>^{\bot}, then <y,w>=0, for all w\inS.
Hence y\inS^{\bot}.
Hence <S>^{\bot}\subseteqS^{\bot}.
Is it correct? Or should I say "for some w"?