There's lots of good advice on here.
Hopefully the statutory KCSIE guidance that comes in from 1st September will help to focus the school on their safeguarding obligations. This guidance signposts schools to both the Cass Report and the Gender Questioning Children guidance and is the key document around which everything centres for schools.
Schools are in a really difficult position because they've been left without clear statutory guidance until this document was (pre)released. It's still not live yet, so remains at risk of change but hopefully not 🤞
Frustratingly, non of the other guidance is finalised, some is non-statutory, and there is no clarity on what will happen. However, if they start from the safeguarding principles of the forthcoming KCSIE guidance, the rest should fall in to place much more easily. I'm no expert on safeguarding (I'm just a parent who is trying to navigate this) but hopefully it should also be obvious to them that under their statutory safeguarding obligations, listening to the parent who is advocating for a Cass-aligned approach is much more appropriate.
OP, in terms of keeping the conversation open with your daughter, I took the "differential diagnosis" approach to working through my daughter's distress with her. She already had an autism diagnosis, but the same approach applies without it. If you've ever seen House with Hugh Laurie, this was the essence of how I did everything: I looked at the possible causes of her distress and started "treating" the most likely. As she is autistic, it was most likely that she didn't like her changing body, was reacting adversely to the change itself and had sensory issues (breast development and periods). She was also badly bullied because of how she reacts in social situations and told me she saw girls as "weak". She still wants to be really strong and has previously said that this was so that she could beat the bullies. In other words she was rejecting girlhood, which is a common theme regardless of the root cause. I'm not going to support her to do that but I can support her with some of the things she finds distressing about everything. I can also talk positively about fitness and strength using good female role models e.g. the Lionesses and Olympic athletes (she really loves playing and watching sport).
Using sports bras and getting her on the progesterone only pill have been a huge help. She did briefly use tape for her breasts (she still really hates them, but thankfully got fed up with the tape and accepted that it was very expensive compared to sports bras) but reluctantly accepted my terms that it had to be sports tape, not trans tape. I really didn't want her to tape her breasts at all and explained to her that my reluctance was because it represented moving away from being a girl - when she originally asked me, neither of us had heard of sports tape and she had found a trans tape product that she wanted me to buy for her. Keeping the status quo while she works through her distress has been my line the whole time e.g. when she was thinking about changing her pronouns I said she couldn't change anything until she had been on a journey of understanding herself better. Luckily we had access to an open-minded paediatrician who supported my approach to treat her distress as autism-related first. It was this paediatrician who helped us talk through the breast-taping issue when we reached an impasse on it. We covered the topic fully, including the physical risk side, with the help of his mediation. He was the one who suggested the sports type idea.
She's got short hair and uses a neutral sounding (nick)name at school. Many girls and women have short hair and neutral sounding nicknames. We did originally try and adopt her nickname at home but she eventually told us that we may as well go back to using the short version of her name (which sounds like a girls' name) because it was probably easiest. The interim Cass Report was out when all of this was going on and I did explain to her that it was an NHS report that made it clear that "social transition is not a neutral act" i.e. the NHS is recommending that we don't do it because it can lead to a fixed destination. She did accept this, albeit reluctantly, because we were still addressing the things that upset her most about her body.
In parallel we've been working on her mental health, partly through CAMHS and partly by ourselves. We've been focusing on getting her out and about, by gradually stretching the boundaries that result from her intense anxiety. At the height of her mental health crisis, she didn't leave the house and pretty much lived under a duvet for several months 😔
It's been and remains a really difficult conversation and I haven't always got it right. It's been so important bringing in other adults to support her, so that she's not just hearing it from me. Having the school onside has been a key part of this - securing this hasn't been easy and it's not always been obvious that it has been moving in the right direction. I can't imagine how difficult it would be to counter the information that your daughter's dad will be giving her but hopefully there is a way to keep her grounded and you can find people who can support you and your daughter 🤞🤞 💐