Dd is ten and has to go to school as an evacuee tomorrow she will take no advice re the packed lunch or clothes and so having spent a large part of the day stressing over it I have told her to take or wear what she likes. So go on dd off you go with your war time lunch of fairy cakes a tuna sanger and an orange dressed on your wartime outfit of a modern skirt and top grrrrrr. What a day <watches baby walk past with mop bucket on one foot> roll on bedtime for all
YANBU, stop stressing yourself about it, let her go as she will and when she realises you were right she will be rather sheepish as the only child dressed in modern clothes etc.
I learnt some years back it's not worth me getting stressed over something the DDs are supposed to do, as long a is have offered advice and made sure they have access to whatever they need, if they say no, he'll mend em.
sometimes you just have to choose your battles. she will soon see the others and wish she had listened, if not, meh what does it matter, I suggest a glass of wine and a bath if you are able. early bedtime for all?
Actually, though I see your point, I think it is a bit unfair on her teacher/ Rest of the class if she doesn't join in. Bound to be other children who will say "but xxx didn't dress up" too, especially it they were reluctant to. I would make her wear uniform if she won't dress up, definitely don't allow home clothes.
Oo when DD did that we had to make a gas mask box as well!
Actually she may not be the only one who doesn't dress appropriately. DDs class all did. But where DH works there is a venue just down the road where school parties go to do this and several times a week he sees crocodiles of children on their way. He says it is interesting that some schools have gone all out to have everyone, including teachers, enter in to it and dress up and others where very few seem to have bothered at all.