Do you have to be an anthroposophist to teach in a Waldorf school?

No.

(And, to answer the question in Comments below, it depends on the program in which you're getting your Waldorf teacher education--they vary considerably and there's no standard or template--and it also depends on what your definition of "studying anthroposophy" is--if you mean reading Steiner's work in areas outside education, that's one thing. If you mean reading any of Steiner's works, including those in education, that's another...)