Hang on - I know it's scary, but there's nothing the police CAN do until something has happened, they can't go around hassling people for what MIGHT happen, much as I'm sorry to say. The school know about the situation, and this girl obviously knows that her card is marked. An alarm is a good idea, so is self-defence classes. Tell your daughter to try and make it so that she's not alone leaving and going into classes, there's safety in numbers, or get a teacher to walk out to the car with her at leaving times. I wish you good luck