It does seem that it is school related , have you asked the teacher, also what does your daughter say, could there be an older child bullying ? It seems that whether or not you trust the school you should find out more form them .something is upsetting her, she cant 'work through emotions till you find out what is happening. Has she had any recent changes in her life , move house, new sibling, loss of anyone , walk in her shoes and see what is happening .good luck