At my sons' school they change in the classroom all together right through year 6. I agree with the consensus that they should change separately from year 5 onwards.
If there is no other place to change, could not the girls be asked to sit quietly in the hall while the boys change and then have them swap? Yes, it would take more time, but if given something useful to do, like practice spellings in pairs, it doesn't need to be wasted time.
I grew up in America, where children didn't actually change for PE until they were 12 or 14 anyway. We did PE in our white school shirts, just changing our bottom half into shorts. We girls pulled a pair of shorts on under our skirts and then took the skirt or pinafore off. No modesty issues at all. Boys changed their trousers, but the shirt tails covered their pants, and now that they make boxers for boys, they could be completely covered. (A very selfconscious boy could even wear a pair of small pants under the boxers.)
Do they really need to change for PE? They play just as hard in the playground at playtime and we don't worry about their uniforms -- why do they need special clothes for PE? Some schools even have soft collar uniform shirts, which other schools have for PE. Where is the logic in that?
I say have them pull on shorts or trackies, take off their ties, change their shoes and be done with it. Less time wasted changing, no embarrassment for the children, and less stuff for parents to remember and wash and haul back and forth.
If schools have summer dresses for girls, they can choose to either forgo the dress on PE day in favour of a skirt and blouse, or deal with taking it off. If a girl is selfconscious she will be happy to have a modest option.