Montrealmum ,
Reading that site I would say that by implication boys must wear long trousers and a shirt or top with sleeves . (IE no shorts or singlet type tops )
So your daughter (it would seem, and you will check ) has an additional obligation , the headscarf, to go on the trip. However , parents of boys would , if they held as strong opinions as you, still have to decide whether they wanted their son to go on a trip where girls were treated differently - so to that extent I think the headscarf is a red herring..
FWIW , I think that you are very wise to allow her to go even though reluctantly and it jars with your deeply held beliefs.
Firstly , she will get the opportunity , if she wishes, to ask the guide about dress for Muslim women . If she is polite and respectful I am sure that she can get across the point that she is being respectful of their beliefs by wearing a scarf and not hypocritical. In turn I am sure they will respect her for doing so. (Sorry got caught in a vortex of respect there)
Also, as far as I see it , we do not live in a single issue world. Yes , equality is important . However Islam has historically been an important force and is (religion aside ) an important factor in world politics. It would be a shame if your daughter could not explore the religion and talk to people who practice it , because of an issue about wearing a headscarf. I knew I was getting old when I stopped thinking there was always an ideal option , just sometimes a least worst one. Now that you know the rules I think you are sensible to compromise on the headscarf to allow her gain some insight into this religion.
I think, given how strongly you feel , you would be better as I think you and PPs have said , to explain to the teacher about your disquiet and suggest the girls have a follow up discussion in class about the trip , to include thoughts on the headscarf. Also , you will be able to have a discussion with DD about it where you can explain your views and she has some understanding of the other point of view. She can then make up her mind.
I personally would just do it for the sake of showing respect for people's beliefs . You clearly feel more strongly. I don't agree with you ,but I absolutely respect your right to your opinion. I think had you decided not to send her , in my opinion only , you would have been throwing the baby out with the bath water.
I hope she has a productive trip.
SE