OP, my dc suffered from trauma five years ago and although is fine now, the school said they had never met anyone quite like him when he first joined them - some of his behaviour was quite extraordinary - yet throughout they have been amazing. So the chances are it is the school failing. One thing I did which really changed everything was to write out everything relevant, including flashpoints, strategies for keeping him calm, and I sat down with his teacher and the SN person and went through it and they followed the strategies. I do think that it was the fact that things continuously improved, quite dramatically at times and otherwise steadily with some fall outs, that made the different. At the same time as suggesting strategies, I let a lot go too, and spent a lot of time at home going through things with my dc developing his resilience and social skills and prepping him about how to deal and encouraging him to cope.
The above is something to bear in mind in the short term with this school or to do with the new school, but at the same time based on what you have said I would also strongly recommend that you talk to a clinical psychologist to get input about strategies you use and and how to manage the school. This video illustrates how a good clinical psychologist can help you with the school - don't be put off by the narrative, it is worth watching all the way through. Whether trauma or attachment or other, clin psychs with trauma expertise would be able to give you strategies and help with communication with school.
accessed by very smart and insightful adoptees who are also sadly very angry and often copy and paste things onto their SM pages this isn't the point of the thread but I am an adoptee and I'd like to say that I have not accessed closed groups or posted anything on SM (at all, as I don't have any SM) and I don't think this is what the OP was worried about, I think it was the recent thread on MN where non adopters were being very critical she was referring to. OP, if so, I don't think you need to worry for this particular thread and an awful lot of non adopters will sympathise here - it isn't just adopted children, it is also SN children for other reasons, G&T children very often as their control centre is often slower to develop, and all sorts. In relation to angry adoptees, a recent independent enquiry found that far, far more research was needed into how adoption was done in all respects including implicitly parenting, and so it is a thing, but not relevant here, and if anyone wants to challenge that statement please start a new thread.