Miami (AFP) – Shots ring out in a Miami school classroom. Inside, a dozen students, their clothing stained with what appears to be blood, desperately scream. A security agent walks down the hall, more shots are heard — and a young man is swiftly knocked down. It’s a terrifying scene — but, thankfully, it is a simulation. The bullets are blanks, and the blood and wounds are fake, as are the screams and the guns carried by the dozens of police officers taking part. The teenagers are volunteers helping police to fine tune their reaction to school shootings in the United States, once again under…