as Michael Moore points out in the movie, there was no footage of the first plane hitting the Tower until later that evening.
Bush claims he saw it before he entered the class room. The only way that he could have seen it that way he saw it is if he saw it live, which he could not have done.
The only other explanation is that he saw a tape of the second plane hit after he left the classroom and got the two confused, but he specifically says that he saw the crash before going into the classroom, and that Andrew Card came in to inform him of the 2nd plane attack.
Likely he saw a tape of the 2nd attack after he left the classroom, and he's either intentionally or unintentionally mixed up his recollections. It was a rather hectic day for him, so it'd be understandable. Or else, as I suggested before, it's just dumb bravado on his part.
Either way, it's really nitpicking to even make a big deal about it. It doesn't matter. He's told many greater, more significant lies than that one. I just wish those were as easy to debunk.