Not "never know" - more like "not find out at school why mommy and her family went to 12 funerals in fall of '01, why mommy still cries when she sees video of the day and wonders if those falling bodies in NY were her friends, why mommy - 14 years later - is still finding out about acquaintances from her childhood who died that day." My NJ town was absolutely devastated on 9/11. I intended to tell my kids about it, but it snuck up on me and I never thought the teacher would have brought it up with first graders. Bad on me for assuming. And I have a pretty good idea of how the conversation unrolled because I actually talked to the teacher about it. My continued anger is that the teacher thought it wasn't that big a deal. To her, 9/11 was as real as Pearl Harbor. For me and my family, no.
PP, I am sorry for your loss. Many families in this area were also affected by 9/11--mine was--to include witnessing the actual event and knowing well someone who was killed. However, you cannot protect your child from events that occurred. I doubt that the teacher shared gruesome details, and I imagine she just stuck to the basics. It sounds to me like your child may have been better off learning about it from the teacher than from you. Clearly, it is too emotional for you to address it.
Also, do not think that the teacher views it as "just another event." Anyone who lived in this area was affected by it to some degree. She probably meant that it "wasn't that big a deal" to the kids. Like Pearl Harbor was a big event to your grandparents--it wasn't that "big a deal" to the ones that came later. That's the way it works.
If you want it to be "that big a deal" to your child, then you should have told her about it.
Sounds to me that this is about you--not your child.