Can't leave the building? Fine then, I'll tell somebody to activate one of the emergency stations near that building while I call the cops and tell them that I just got a call from an unknown person saying he/she was going to shoot this building up in 10 minutes. If they tell me to get out of the building I'll say that the person also said that if I left it he/she would bring about even worse consequences. If the amount of time left isn't too close to arrival I'd probably try and warn others until it gets to a point where them leaving would probably involve them walking right into the guy (if the police are not already there and waiting for him) and just barricade one of the classrooms.