It was not appropriate for the sub to talk about it.

I was out of the country with my husband during 9/11. My kids who were almost 7 & 2 were home home with my in-laws. Thankfully both my in-laws and my school kept mum about it. (Only watching the TV after the kids were out of the house.) My 7 year old did NOT need to hear about this with us out of the country. We were fine & perfect safe just in Europe. Not easy for a 7 year old to grasp. They are now 18 & 14, have been to see ground zero and know all about it. But I am VERY VERY grateful that my kids school DID NOT talk about this kind of issue.

IMO Middle School & Older it might be OK, but not non-stop in all the classrooms.