There ARE incompetent teachers, but far fewer than you would think, most are weeded out by the rigour of the PGCE and NQT year.
it's also worth pointing out that for most middle-class parents, the situation now is wildly different to when they were at school (this for the UK, I realise the OP is talking about France). When I was at school in the 1970s and 1980s, there were a significant number of teachers who were completely incompetent. They were less qualified (some wouldn't have had a PGCE, some would have a 2 year Cert Ed) and it was at that point both very difficult to fail your PGCE and very difficult to not get appointed after your probationary year: the shadow of RoSLA meant that a teacher who had a pulse was appointable.
But today, the situation is very different. Passing a PGCE and probationary year and getting QTS is difficult, the drop-out rate is high (in some cases for bad, but in many cases for good) reasons and the accountability measures mean that teachers who can't deliver aren't carried by schools frightened of embarking on competency proceedings, as was the case a generation ago. My children's school education is coming to an end, and I've not encountered a single incompetent teacher and I've only encountered a couple where I thought "yeah, OK, but I wouldn't want you teaching my child two years running" (and, notably, they were all older). No teacher under fifty is likely to be incompetent, simply because they would have been weeded out.
With my "child of teaching union activists" hat on I'd say that in some cases schools are trigger happy and embark on competency proceedings at the drop of a hat when support and coaching would be more productive, but in 2015 schools are quite focussed on the interests of the consumers (yeah, as measured by metrics which you can argue with) rather than the producers. That's overall a good thing.
There are far fewer crap GPs, too. And opticians. And any other regulated profession. They've all decided that allowing 5% of the cohort to ruin the credibility of the 95% isn't good business.