Why dont french schools teach RE?

Religious Education is mostly a family matter in France. In a vast majority of schools (nearly all in fact), even private ones, the teachers are state-paid. The schools have then an obligation to follow nationwide guidelines on the curriculum.

Religious education cannot be compulsory, even for schools run by religious bodies. A Catholic headmaster cannot refuse a pupil in his school, on the motive that he is not Catholic. Catholic pupils can be taught religious education, on demand. This is no more popular as it is an additional class instead of time off. This is not popular too, with parents thinking their children have already enough to learn.

However, there are some classes at age 13/14 explaining the cultural differences between religions.