What is the school supposed to do? If they try to discipline the child, the parents will be all over them for abuse. If they don't do anything, and another child gets hurt, parents will be all over them. So they call the police/social services, who come and do their jobs and the parents are all over them.
Of course, calling the parents to come and take care of their child might not do any good, either, since so many parents believe their child is a little angel and the school/police are overreacting.
Was it necessary to handcuff her? Maybe not, but a slap on the butt to get her attention wouldn't have gone over very well either. And remember, the police do have procedures, and if they put those procedures aside and something happened to the girl, the whole community would have been on their case. Seems like a no-win situation for all concerned.