Schur-Weyl duality over finite fields

We prove a version of Schur--Weyl duality over finite fields. We prove that for any field $k$, if $k$ has at least $r+1$ elements, then Schur--Weyl duality holds for the $r$th tensor power of a finite dimensional vector space $V$. Moreover, if the dimension of $V$ is at least $r+1$, the natural map $k\Sym_r \to End\_{GL(V)}(V^{\otimes r})$ is an isomorphism. This isomorphism may fail if $\dim_k V$ is not strictly larger than $r$.