Even though I'd kind of decided nothing could penetrate this idiocy, I do want to point out that previous to 9/11, people had tried to use planes as weapons to kill the president on a few occasions. A couple of times they totally missed the White House, and at least one time they hit it, but didn't do much damage to it. The World Trade Center was a very large building that was easy to build straight line acceleration towards. You didn't need to maneuver at all to hit it. Also, as a skyscraper, it was very reliant on its support structures to keep it up. This is also why the damage to the Pentagon was relatively minimal in comparison...you could go straight line acceleration to it, but it's not a very tall building, so it's not going to collapse in the same fashion.
Due to the surrounding urban areas, you can't go straight line acceleration to either the Booker school or the White House. There's other buildings around them, and the White House fence has been an obstacle in at least one of those incidents too.
What does this mean? Well, it means that the Secret Service has incidents to examine of planes trying to attack the location of the president and has a plan formulated. In a building like Booker School, they knew that wasn't a problem.
Now, they also have incidents to examine of escorting the president out to his limo. Even the brief space between the building and the limo was enough for Hinckley to shoot Reagan. And in the limo was a place that Oswald or the grassy knoll guy or whoever could shoot Kennedy. One of the most famous political assassinations of all time was in a vehicle driving around, Franz Ferdinand. And then he'd have to board his plane, meaning being in a transportation hub of some sort, which was a good enough place for a guy to kill Garfield.
They know where you are, so they know where your limo and plane are, and if they're attacking you, going there is the best place for you to get yourself shot. Those are the weak points, and so it's not a coincidence that that's where most assassination attempts happen.