To answer pie+mash's question - we all feel exactly the same, that political correctness is being used inappropriately.
The notion of stopping / removing something 'in case Muslims are offended' is an utter nonsense. Apart from the press-grabbing fundamentalists, i am sure that Muslims are happy to co-exist with other faiths, and would be perfectly happy to see Christmas lights in what is, nominally at least, a Christian country.
With regard to your question enigma, I would by-pass the school and contact the mosque directly, and find out exactly what is required by visitors in terms or appropriate attire - i wonder if the school is being over-cautious in requesting full head and face coverage?