I've know boys and girls changing together for PE in primary school is often discussed on Mumsnet and I thought anyone having to discuss this with their own school may be interested to know that the NSPCC have just released best practice guidelines which recommend that boys and girls be separated either by using separate rooms or where that is not possible to use screens or furniture as a divider.
www.nspcc.org.uk/globalassets/documents/information-service/schools-factsheet-best-practice-for-pe-changing-rooms.pdf