Columbine?

I believe so. One of the columbine survivors is the director so I'm sure it's told by his point of view.

The film uses the words "inspired by" rather than "based on", I'm sure because the director took creative license to make the story different. Like the other poster said though, the director is a survivor of the shooting at Columbine High School, so I imagine a lot of the story especially the actually event is told from his perspective and what he experienced during the attack.