Dieu, why is she so shy around boys and men?
Some posters have asked that you try to get to the bottom of this -- perhaps there has been some sort of incident. I think that is very good advice.
I am one of three sisters, and went to an all girls convent primary school, followed by a mixed equivalent of a comprehensive (in Ireland). I am very glad I had the experience of going to both schools. I feel if single sex schools are to exist, then they should exist at primary level only.
Also, I have four DDs, two of whom are redheads, and one DS. They have all gone to mixed sex schools from kindergarten on. In their schools there is a zero tolerance policy on sexually based harassment, whether directed at girls or boys, or aimed at shaming sexuality, sexual orientation, etc. There is also zero tolerance for body shaming of any kind. I think you should bring up the issue of any shaming, harassment and bullying that has come up, separately from the residential trip. You should march in to the Principal's office asap, armed with screen shots, etc, and demand to know what draconian solution is going to be put in place to stop this dead in its tracks. You should also march into the office of the Principal of the boys' school. It is completely unacceptable.
It goes without saying that none of this should be tolerated on the residential trip either, and you should be presented with written guidelines for behaviour along with sanctions that will be applied. But in case the schools are unaware of the ongoing undercurrents, they need to be informed, and students should be able to rest assured that all students are expected to behave in a civilised fashion at all times.
I am concerned that your DD has closed off discussion of going to another school. I understand that she is a nervous, anxious sort of girl. I wonder if anything has happened to make her as nervous about males as she seems to be but that is a different matter... But I think you need to get to the bottom of why she is refusing to contemplate going to another school, even though she has to negotiate a certain amount of bitching at her own school as well as the fact that the school is 'paired' with a boys' school that seems to foster a misogynistic culture. Overall, there seems to be toleration of a level of nastiness that is making her life miserable there.
It is problematic when someone is willing to cling to the trauma she knows rather than deal with, as she may see it, potential or unknown trauma. School is not meant to be one continuous episode of hazing. I would try to find out what she sees as the tradeoff for the unhappiness and why she values it.