The Secret Service at Booker Elementary: The Dog That Did Not Bark

I think Michael Rivero should go a bit easy on this issue:

With a supposedly unknown number of planes flying over the nation and crashing into buildings, with Bush's presence at Booker Elementary announced in the media three days in advance, and with an airport just 4 miles away, how did the United States Secret Service know for a fact that Bush was safe where he sat reading about goats? How did they know they did not need to throw him into that armored limousine and start driving to foil an intercept? How did they know that by keeping Bush in that room they were not making targets out of all those teachers and students? How did they KNOW they were not targets?

It seems to me that too much stress is being placed on the fact that it was foreknowledge that prompted the Secret Service to act the way they did, when it could just have been an inability to respond correctly to the situation because of various reasons ("never attribute to malice what can be adequately explained with stupidity" comes to mind).