I don't find the question as easy as some here because it falls under two 'rights' in my opinion. The right to wear or not wear what you want and not be dictated to, and the right of others to be able to see who it is they are with. For me it is deciding the priority, since the two clash.
I've contributed to threads on this question before and have reluctantly concluded then, and still think now, that the right of others to not have someone hiding amongst them is more important.
In the case of school children I can see that could be considered harsh, so I'm not 100% against making kids an exception, (although 6th form is getting on a bit for concessions) but since they eventually have to live as an adult in an adult society it is probably best to learn that hiding like that isn't on.
And I would hold the concern that to do so when young might even encourage isolation / lack of integration with the others, maybe resulting in stunted social skills and possibly even issues of self worth, given that hiding is not the 'norm' here. So it is probably not a good idea then either.
In any case, was the school rules not known in advance ? Surely this should not have been a shock to anyone ? What is all this about wearing veils ok, but not a niqab in the 6th form ? Is that a new rule ?