Well it would seem that they are specifically afraid of rising religious extremism, rather than extremism in general. My only query is whether or not the measures they are taking will make any difference.
What constitutes extremism, what about green peace advocates, who in the past couple years have escalated to commiting felonies such as vandalism and arson. Basically, there is no reason that any of the examples I have given should lead to greater extremism UNLESS they are oppressed for what they are saying/believing.
I would not have a problem with France doing that, if it came out and said, " we are banning the support of any activist group, religious or otherwise, in the domains of our schools". That would atleast be consistent, ableit a little off the deep end.