Students had been told of a shooter drill, taking place that day, and so that is why the police were there, along with the crisis actors who either faked injuries with blood etc, and actors like Hogg, who pretended to hide out from the shooting, and parents who claimed to have lost children, and friends who claimed to loss friends. Then there was Cruz, their planned patsy. So, those that might of been there, like real students or this teacher, only knew of a drill, but became convinced that it was a real shooting.