Things I Tried
Googled various combinations of "Thousand Paper Cranes" / Sadako / school / kids / children / read / teach OR taught / book
Googled "Sadako" OR "Thousand Paper Cranes" in livejournal.com only, hoping to get something from one of the book, reading, or education comms.
Did a "Find on Page" search of Amazon reviews for "school", "7" (thinking someone might have mentioned reading it in the '70s), and "elementary" (the age my character would be when she reads the book, if it fact it was being read in schools in the late '70s/early '80s), and checked the profiles of the few people who mentioned reading it when they were the right age to see how old they were in the right time frame (one is about ten years too young, one wrote as a guest, and the third, I can't find any age info for). I also did the math for the two people who mentioned in their review how old they are; they also were both too young.
Thanks to anyone who can help!
Edited to add that the book doesn't actually have to have been "assigned", as it were. I'm more interested in just whether or not a kid would be familiar with it. So if your teacher read it to you, or it was in the school library, or even on display at your local public library, that would all be fine. :-)