1. First graders are not members of a terrorist organization.
2. For the terrorists to attack the elementary school with aircraft, they would have to know it's location and be able to find and target it from the air at about 400 miles per hour. As far as an attack on the ground, security doctrine requires that you not have the president sitting in the open for long periods, which means you only move him to the motorcade when the motorcade is going somewhere and he will be able to move to the next step as soon as you arrive. That brings u8s to the next reason...
3. Air Force One does not just leap into the air. Safe flights take time. Why move the president from a target that would be impossible to attack (the school) to a target much easier to attack--the tarmac at an Air Force base with the world's best known 747 sitting on it?
Well put, Silverback.