I have experience of this with a boy in Y8 at my mainstream secondary. We are currently in the process of an emergency review where school and home agree specialist. They were offered specialist at transition but declined and now regret this.
Has your child got an EHCP?
I was told by CAMHS 70% of students with PDA are out of education.
What has been most successful for us is: a robust reward system including home in this. Offering choices for everything. I sometimes tell them I am going to do something, knowing they will say no. For instance today I said, I will scribe for you today and he replied no I will write for myself. That worked well! He does try to negotiate everything, we do allow some negotiation within reason. Sometimes it doesn’t really matter to me, i.e. can I sit there or can I use a pencil instead of own so we agree. We have had to adjust the timetable. I wonder if primary if you could have a list of tasks to do for the day but they could pick when they do them? Presumably if they have an EHCP they have support to enable this?
We had a visit from ASD Outreach and they provided some really useful strategies. Have you access to this?
It is really tough. I suggest you document everything and keep parents in the loop about whether the placement is likely to fail etc.