well he was interrupted reading my pet goat to a a bunch of school kids and after being told that a plane just flew into one of the twin towers he said OK, and then went on reading with the class as thousands burned up when the second building was struck.
He maintained the illusion of normalcy as best he could. If he'd dropped the book, screamed 'ZOMG!!!!11!!!' and ran down the hall, what would he have achieved other than possibly worrying the kids?
You are, of course, using hyperbole to make a point...I do that myself. I recall, however, a failed television show starring Gina Davis as the first female President and in the show they set up a similar situation where the President was addressing school children and then was informed of an emergency situation. On this fictional show, the President excused herself from the children. Now, before you go thinking I confuse TV with reality, I'm well aware that it was a TV show, but it looked perfectly natural for the President to have excused herself.
President Bush handled that situation badly, especially since he was on camera the whole time. He's the President and can be excused for being rude to kids reading a book. They get over it. They would have found all soon enough why he left.