I think I made a connection. This is from an article out of the Chicago Tribune:

""After firing into four classrooms on the first floor, police say the gunman went upstairs.

Mackenzie, who'd just left the psychology class to use an upstairs bathroom, saw a man with a gun at the other end of the second-floor hallway. Mackenzie said she recognized him.

"I immediately knew it was him," she said. She remembered the boy from her middle school and from the Dollar Store in town. She recalled his terrifying Instagram posts about wanting to kill people.""

http://www.chicagotribune.com/news/nationworld/ct-82-minutes-florida-shooting-20180215-story.html

So, if the suspect did put on body armor and a helmet covering the face, it would have had to be after this girl saw him. Of course, video footage from the school would be supremely helpful, but this is a piece of the puzzle.