I'd say that if you want them to have any academic qualifications then you should stay in the UK until they have done A levels and gone through the UCAS process. Professional qualifications here require a good grasp of French to succeed even at a very basic level, eg CAP.
There are international schools in France where you can do A levels, but for all sorts of reasons they aren't as reliable as a good school in GB, especially for people who speak no French.
I'm sorry to sound so negative, but it is unrealistic to expect non-French-speaking people that age to do well at a normal school here, they will, unfortunately, end up with very little choice of what to do next and it isn't something I would recommend at all.
I am sure you will have people telling you they haven't passed an exam in their life and have done fine, but times have changed and especially in France you need paper qualifications to do pretty much anything.
I teach in lycée and have several children at various stages in the system, this is my opinion as both a teacher and as a parent.