I was in 8th grade history class. I was scrambling to finish a worksheet that was supposed to be due in a few minutes that I neglected to do as homework, because I sucked at time management. The teachers were all called out of the rooms, so I was psyched that I might actually get my worksheet finished in time. When my history teacher returned, she had one of those TV carts from the library, so that only put me in an even better mood. Got my homework done on time AND it's a movie day? Sweet! She also sent one of my classmates to the office, which I didn't really think much of at the time.
Yeah... then she put on the news. We all just kinda sat there in shocked silence. I don't recall for how long we watched, but the school sent us all home early, and it was the most eerily quiet bus ride ever considering it was full of middle school children.
I don't think I really knew how to process it at the time, so I just kinda went into normal after-school routine. I booted up my PC and started playing some StarCraft: Brood War multiplayer. At least, I tried to. Within moments of checking out the Use Map Settings section, there were already World Trade Center themed maps appearing. That kinda made me a bit sick, so I called it quits and worked on a school project instead.
I found out later that the classmate that was sent to the office was removed from the class because her dad was a pilot who worked for American Airlines, and he was flying that morning. Thankfully his aircraft wasn't one of the ones hijacked, though.