To teach in a French "public" high school (= not private!), you need to have a formal teaching qualification, the CAPES (Certificat d'aptitude au professorat de l'enseignement du second degr�).
You could become a language "assistant" without this, but you would only work few hours, and the posts are usually given to foreign students of French on a gap year.
Alternatively, "private" schools (=mostly catholic) do not require the CAPES and you can teach without it provided you have relevant qualification/experience (I think).
Have a look here for some info on the French system:
http://www.education.gouv.fr/