I'm not aware that I mentioned the first plane...
As others have pointed out, Bush has stated that he saw the first plane hit before entering the classroom.
His words were, ""Actually I was in a classroom talking about a reading program that works. And I was sitting outside the classroom waiting to go in, and I saw an airplane hit the tower -- the TV was obviously on, and I use to fly myself, and I said, "There's one terrible pilot." And I said, "It must have been a horrible accident." But I was whisked off there - I didn't have much time to think about it."
It was *after* this that he went into the classroom.
I agree entirely that most people thought the first plane was a terrible accident until the second one went in. I certainly did.
The video records how long it was between being told the second plane had gone in and 'We're under attack' from Andrew Card and Bush doing anything.