Essex Girl is quite right. The idea of school uniforms originated to identify the school, so as the school could be contacted if any student misbehaved or even did a good deed. It was also a symbol of pride in one's school, especially if the school had a historic, or good reputation to uphold.
In later years it prevented some students from attending class in the latest trendiest expensive designer clothing, whereas students from families that could not afford the latest styles would not feel embarassed. The uniforms are an identity/equalization instrument. I am sure this was explained to the emigrant students and if they do not want to abide by the rules of the school, then go to another school. Period.
I guarantee that any french student attending school in a mid-east/ african country would have to abide by the standards set by the school or else they would face some kind of disciplinary action.
I also do not go for the poverty angle either, in almost all cases, any family that is able to leave a third world for whatever reason, are normally quite well off, otherwise they could not afford to leave in the first place. Many of these immigrant families I am sure had servants etc in the respected homelands. When they get to a new land and find they are all on a level playing field, their lives are changed and some cannot accept that. They feel the new land owes them something , they feel like they should be able to live like they did in the old country, well, sorry NO. Get out an work like everyone else, there are no free rides anywhere anymore.
Of course rioting etc takes the focus away from the real problem, they just don't want to assimilate, they just don't want to have to start over, they just don't want to do what emigrants have been doing for hundreds of years, that is get started somewhere and work your way up without relying on the state for hand outs.

We have a similar situation here in Canada, almost daily the shootings in Toronto are blamed on poverty and disheartened youths who have no money and nothing to do. And yet on a daily basis I pass store after store with advertisements in the window looking for staff. These people only make up a small percentage of their own society and race, Many other youths from similar backgrounds and just as poor but do not go around rioting and shooting, instead they get part-time jobs, they attend school and one day they will reap the benefits while the others will still be complaining about this, that and the other.
If I was the president of France I would go on T.V. and give the message loud and clear. rioters will be hosed with water cannon, will be sprayed with tear gas, will be charged and sentenced and NOT BE treated with rubbber gloves, and at the very next opportunity make good on the warnings,
Can you just imagine what would happen to a bunch of french citizens, living in Somalia or some other similar place if they took to the streets burning and rioting, They would be locked up and the key thrown away.
I am an immigrant myself and I know first hand what it is like to go to another country,, with no job, no friends and almost no money so I am not altogether unsympathetic to their plight, but they don't have the right to burn, riot and cause the damsge that is a nightly thing now in France. The fact that it has spread from town to town, appears very much like the rioting etc is in fact deliberate, planned , and ordered from one source.