I was sitting in my high school class for EMT (emergency medical technician), a pilot program offered to junior & senior students in replacement to 2 marking periods of health and gym (more than enough of both during the training).
The class was in 2 parts. First part was before normal class hours, beginning at 7:30 am. First period was officially at 8:20 AM. The second part was the 'first' period class timeslot.
We finished our lecture for the day and sat waiting for the bell to ring at 9:10 am. Continued on to my boring 2nd period class of sociology. By then the first tower was hit already at 8:46 am. We hadn't heard anything at all yet.
Went onto homeroom (yeah, my school was and still is fucked up that homeroom comes between 2nd and 3rd periods). Still no one knew anything or was said.
3rd period was my drama class (we read plays in classroom *rolls eyes*). The teacher wasn't there yet and it was no biggie. He was routinely late as he was also the school's activities director and got stopped by students in his office or hallway. Usually he was right outside the classroom talking to someone (and man did he love to talk!)
Twenty minutes into class, just before 10:00 am, he came rushing in. No words, just threw the television on and flipped to one of the few local channels that the school received. Before we realized what was going on, we were watching right as the first tower collapsed.
I could swear it got so silent that you heard people's hearts beating in their chests. I don't remember what happened right after that but when 4th period came, I skipped lunch and went to the office. Any student that was under 18 years, were not allowed to leave the school without a parent. Not even if the parent authorized over the phone. So I was running around the school, getting students and passing messages off from call ins to kids. 5th period was my study hall so I wasn't bothered. I worked it in the office anyway so I was covered. 6th period I was still running around and had to get a student from the music room which was right across from my drafting class (yes, my senior year was full of Mickey Mouse courses) and I was able to pop my head in to let my teacher know that I wasn't cutting or anything, that the office was using me still. He waved me off, saying I was fine and guaranteed to pass with an A already (first day he gave extra credit for the marking period and I had it in two days after, well past his expectations to know that I will top all his assignments).
7th period, my mother showed up to get me (I didn't know she was even coming) and we rushed home because she told me that my uncle had been scheduled to do some repairs for an affiliate office from telecharge in one of the towers that morning. We hadn't heard from him yet and she and my grandmother were freaking out.
It wasn't until hours later that he finally was able to get word that he was fine, trying to make it downtown to the ferries where they were taking people over the river to Jersey City and Hoboken in the evacuation.
He had to walk from there all the way up to Weehawken almost to get to where my other uncle was waiting (traffic and shut downs of the tunnels and all areas like that).
He had missed it because he overslept that morning and was late into work. He said that when he walked out of the Port Authority, he saw people screaming and freaking but he didn't know or think about it much. When he got to the office, the doorman told him what happened and until he got upstairs to see the TV and hear on the radio, he didn't believe a thing.
I know EMS people I used to volunteer with were working that day and told me that when they walked in to find their crew watching the news, one girl who didn't know yet asked 'What fucked up movie are you watching?' before realizing it was the real news.