Concerning 9/11....how can this be?

Here is the question none of the defenders of the "official story" can answer: with a supposedly unknown number of planes in the air all over the eastern part of the country, with bush's presence at Booker Elementary announced in the news three days prior, with an airport only four miles away, how did the secret service KNOW that it was safe to let Bush remain seated in that cl room reading about goats? How did they KNOW they were not making targets of all those women and children by keeping them there? How did they KNOW that Bush was not a target?