Yo-Yo Girl Cop (2003)

A police detective working undercover at private school Seisen Gakuen is blown to pieces on the streets of Shibuya. The cop had been investigating students frequenting a popular underground website named 'Enola Gay', where information on everything from bullying to bomb-making is exchanged.
One week earlier, a strange counter on the website began ticking down to some unmentioned event… Meanwhile, a young girl only known as 'K' is brought back to Japan from New York against her will, and is assigned the codename 'Asamiya Saki' by the National Police Agency. Her mission: infiltrate Seisen Gakuen.