Hogg has given several conflicting stories about this. In his earliest pronouncement he said he was at home when he heard the news of the shooting and grabbed his camera and ran to the school. By the time he got there, it was all over.
He never said he was at home before the shooting. Watch the video. He said, "On the day of the shooting, I got my camera and got on my bike and rode as fast as I could three miles from my house to the school..."
Other reports then claimed he was not at school before that. Those reports were wrong.