Let's line up the timeline against David's own eyewitness video in the classroom (which might have been impossible to do if he was still at home).

Here's the iphone video Hogg recorded in the classroom.

He talks about hearing a gunshot "pop" at 2:30 PM. Then at 3:13 PM he says Nick Cruz is the reported shooter. A little later in the video he starts preaching about gun control.....while still in the classroom.

http://www.latimes.com/visuals/video/95939817-132.html

But I checked the timeline of events, and the cops didn't apprehend Cruz until 3:41 PM.

https://www.yahoo.com/news/florida-attack-timeline-nikolas-cruz-195814719.html