What a horrid (but all too common) situation for you all. Do what neo says and contact the local nick. There are a number of potential charges here assault, threatening behaviour, harassment. If you make a complaint it is possible that this girl will be offered a caution (if she admits it and if she has no previous), if not, the Youth Court is likely to be where it ends up. IF it goes this far you could make representations to the Officer in the case about bail conditions (ie non contact etc) and if a conviction results (which would necessitate your daughter giving evidence in court, unless the bully pleads guilty) there is the possibility of a restraining order. However, it is fair to say that all this will be stressful for you and your girl.
If the police are reluctant to get involved you will have to insist on making a formal complaint - the more detail you can give about dates, places, times, witnesses etc the better so get your daughter to keep a note.
I suspect however, your local bobby will be more than happy to help - usually the police would far rather prevent stuff happening at a later stage by nipping it in the bud.