OP i'm a primary school teacher and i have taught several children both during my training and in my class who have attachment disorders.
i'm normally the first to stand up for other teachers, but in this case the parents, the children, your child and you are not the problem. the teacher is.
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She is not just telling you that there has been complaints but that the parents have been phoning each other up. why did she feel the need to tell you this? i've had similar problems and would never dream of saying that to the parents, the only possible outcome would be the parent and child feeling more excluded. why would she want that?
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letting the majority of the children getting changed for a party at school (not on in itself imo) knowing that a child who has social difficulties has not been invited! i'm sorry but she has no understanding whatsoever. terrible decision. it instantly brings doubts to my mind about how she is helping your child to make friends in the class.
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i believe i read that there had only been a couple of incidents that were worth the teacher complaining, and to me those incidents do not equal to what the teacher is saying about the parents complaints. my inkling, the 'complaints' are exaggerated and it has probably just been the odd parents saying "just letting you know that xxxx happened yesterday, just making you aware".
i'm sorry OP but i think you should move your daughter to a school that is more understanding of her needs. the teacher should be focusing on helping her make friends not saying daft things that will exclude you and her further. i honestly think they are not used to dealing with children with difficult behaviour and are hoping that you will move her. don't get into a battle with them, for now focus on your daughter and just find the right school for her.
as for the party, probably nothing to do with parents excluding her, more likely due to a combination of an atmosphere between the parents and you, which hasn't been helped by the teacher, and friendships between the children themselves. also if your child's behaviour can be a bit difficult, maybe they were worried that you were just going to drop her off and go and they would struggle with her behaviour.
it might not be a bad idea to say something to the parents. no details though, just gereal conversation. just along the lines of "we adopted xxx this long ago. she's finds some situations difficult but she has come on leaps and bounds and i am so proud of her". no need to say anything else, it might make them a bit more sensitive though.
good luck to you and your daughter x