Columbine high school does not appear to be a real school. The students and teachers are all actors. In this video clip from Columbine high school, a student is dressed as an assassin. This is because a trained assassin is used to kill students at schools like Columbine high school. If you can't take a hint, you will be killed like the rest of them.

https://www.youtube.com/watch?v=Rzi6n8eCkUI