Right okay, so the cap appears to be the cap worn by student-officers in the french gendarmerie (a military force charged with civilian duties within France.)

Here's a French wikipedia article about their uniform. I speak French so here's a quick rundown of what it says:

*The uniform is called the Tetra *It's worn by cadets training to be officers. *"Tetra" is a contraction of "tenue de tradition" (keeping tradition). *The cap itself is called the taconnet. *Only the students have the blue plumes. *The tunic has remained reasonably similar since 1871. There are different ornaments and stripes according to rank.

So yeah, they seem to be students, perhaps graduating ones given that they're in some kind of procession with a ceremonial French flag, not sure where, and it looks like the 50's or 60's but of course, that's just conjecture.