I was in middle school at the time. They told us nothing, so I didn't know anything until long after the attacks had actually happened (around 3:00 in the afternoon when I got home, when the attacks had happened at around 7:30 in the morning if I recall correctly).
I remember being very surprised that two airplanes would crash into two buildings, right next to each other. It didn't occur to me that it was an attack until my parents explained everything to me after I'd spent about an hour watching the planes hit the towers on CNN over ... and over ... and over ... and over ...