After reading your posts, I feel that you are mom that loves your daughter very much. But you have never show any sympathy to the boy. Have you think that how the boy was hurt after he was labeled as stupid in school. What the principal did is totally wrong to force the child admit in public it is just a joke.
Regardless what the boy feel or he may not feel it due to all kinds of reason (may be he is not mentally competent), what the girls did is wrong. You should take this chance to teach your daughter how to treat other people instead of busing to escaping responsibility.
Shame on you.