Agree about ISI having no bite. Sorry for suggesting all the things that you've already done OP.
Can they not be legally held accountable re they failed in relation to their duty of care towards your daughter and they failed to follow their own procedures re investigating properly your written complaint against the HT? If they're not going to follow their own policies and procedures then they are not worth the paper they are written on.
Why not post on the legal section on here for advice from any solicitors who are on MN? Worth a try?