But it IS taught. I have taught it, and know other American ESL teachers who have, too.
This usage of the word 'of' is not a big secret. That it's a "typical" difference between British English and American English is actually pretty well-known in ESL circles. At least that's my experience. Look at this Wikipedia article (scroll down to "Time-telling"):
http://en.wikipedia.org/wiki/American_and_British_English_differences#Time-telling