Sitting in a high school C programming class, as I recall. When it became apparent that something was happening, I believe several of us entered a teacher's lounge to get to the TV. Both planes had already impacted the towers, but I did watch them collapse on apparently live TV. I suppose in retrospect it was interesting 'watching history in the making', to use a bad cliche.
For whatever reason, I was neither shocked nor particularly surprised compared to most people. I was, however, personally blamed (jokingly) for the attack at least twice before the end of the day. Some may now say that my accusers were in bad taste. I didn't really care.