I am countering your argument that this law is useful because it prevents arguments in schools. This is both incorrect and lazy.
Right, because there aren't any precedents with, say, banning smoking in schools. If you leave it entirely at the school level then a small number of motivated parents can harass the schools to get what they want. They did it with smoking[1] ("we don't want our children to leave the school grounds to smoke") and they did it with phones ("we want to be able to reach our children at any time"). Being able to stay "that's the law and that's it" is definitely a good way to avoid having this debate every year in September.
[1] http://www.lavoixdunord.fr/213036/article/2017-09-04/fumer-d...
[2] https://bit.ly/2xEDILf (sorry for the minified URL, HN won't let me post the original for some reason)
This is just a terrible argument. Just because a subset of people may be able to enact rules on their own does not mean you shouldn't create consistency at a higher level. Especially if you desire a specific outcome instead of having some groups come up with something completely different.
Also this law was one of Macron's campaign promises. People voted for this and people got what they voted for. Apparently people didn't want schools to make up their own rules.
Arguing that a law is pointless because there are exceptions is like saying that red lights are pointless because ambulances are allowed to ignore them.
Of course if it turns out that most schools end up creating a huge number of exceptions which make it trivial for everybody to side-step the law then yes, it will end up being useless. It's a bit too early to tell though.