I was in high school. I remember hearing about it in English and class thinking pretty much the same thing, it was a mistake and must have been a small airplane.
Next class was TV Broadcasting and we were glued to the TV. I remembered skipping my following classes. When we heard about the Pentagon I rushed to call my Aunt (her son, my cousin) was a Navy Flag Lieutenant at the time and worked in the Pentagon. Found out later in the day his admiral and their staff wasn't in the office that day, luckily.