I was a senior in high school. I was only taking 4 classes that semester, and each one was a little different and somewhat odd.
My first class, when I sort of found out what happened, was Citizenship Issues, basically a glorified current events class. My teacher in that class could tend to be a little oblivious to what was going on right around him, so the situation that played out wasn't all that surprising from that perspective. We were watching a video series in that class on the Constitution, and at this was when most things were still on VHS! My teacher turned on the TV to start the video, and realized it hadn't been rewound from where another class was watching it, to get to the point we were at in the video. He stops the video to rewind the tape, and proceeds to stand there while rewinding the tape, facing the class to talk to us about something we'll be watching in the video, never realizing that no one was really paying attention to him, and was instead watching the TV screen. The TV was on one of the networks, I seem to recall maybe ABC, and they were of course reporting on what had happened so far, and had video of one of the towers burning from the plane hitting it. My blood pretty much instantly went cold, as I never really thought it was anything but terrorism; but at this point, only one tower had been hit. Never even realizing what was going on, the tape got to where it needed to be, so my teacher hit play and started the video, and that was it for about the next 20 minutes or so. No one was really paying attention, everyone was whispering back and forth about what we had just seen. Then the teacher from next door came over, and talked to my teacher briefly. He stopped the video and we spent the rest of that class period watching the coverage. By the time we got back to the coverage, the second tower had been hit, and it was pretty clear this was much more than an accident.
That class got over, and I head to my next class, which was economics. This class waas completely bizarre, because it was like for that hour, nothing strange was going on in the world. The teacher hadn't even heard anything about what was going on, and when someone mentioned it to her, she kinda just brushed it off, and went forward with her normal lesson plan. For one hour, I and everyone else in that class sat there wondering what was going on (this was well before smartphones so not really a way to check).
That class got over, and I went to my next class, which was Creative Writing, and was in a computer lab. There was no TV in this lab, so there was no way to watch what was going on. Our teacher pretty much just let us do whatever for this period, because he knew no one would be able to focus, so I spent the entire class period trying to find information. It took me about 40 mins, or a good portion of the class period, just to get CNN's website to load up, so I could see what was going on. By this point, the towers had fallen and the plane had crashed in Pennsylvania, and the plane had hit the Pentagon, basically all the major stuff was over.
After that class, I went to my last class, which was called Independent Lit, basically you read books of your choice and did reports on them. That whole class period was spent watching TV coverage, and my teacher was crying, because she had people she knew that lived in New York and wasn't sure what was going on.
Then I went home and spent the rest of the day just watching coverage of everything.