I’m wondering the same...
I think in the video he’s talking about when he comes back to school at 6pm that night. If you read the Vox interview (https://www.vox.com/policy-and-politics/2018/2/20/17030294/florida-shooting-survivor-david-hogg-gun-control ) and the Ingraham interview (http://www.foxnews.com/transcript/2018/02/14/student-says-heroic-janitor-saved-many-lives-during-shooting.html ) it becomes more clear. The part that was weird in the video was when he says the thing about this not being another mass shooting. But from those two other videos where he says something similar, it seems he means to say “this can’t just be another mass shooting, something that people soon forget...” So It think he’s not talking about being at home when the shooting starts, but is referring to when he goes back to the school later that day.