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Thanks everyone, he went to school yesterday and spoke to the children in question individually to say he's not taking sides, he's friends with everyone, and the whole thing is pathetic (very proud of him)- everyone took it well and he assures me they are friends again.
Unfortunately the girl is still being singled out, but I have parents evening tomorrow and I will address it with his teacher then.
Just wanna thank everyone though for nice comments it's very much appreciated.