There was a timeline started someplace that said the school doors lock automatically at 9:30. By 9:41, the first 911 call came through.
Most schools start at 8 - 8:30 so that's a bit late to lock the front door. I've thought from the start that he was already inside before the doors locked. If his mother did work there (and that's still confusing) then he'd know what time he needed to be at the school. Reports are that the LE had to break doors to get in so that might be the reason for the broken glass that was reported by children on their way out.