My maternal grandmother had the equivalent to a 5th or 6th grade education (she grew up in the 1920s), and I bet if she were still alive, she could also run circles around the average NYC HS graduate.
Someone posted recently that an American 6th-grade education circa 1902 was equivalent -- in terms of reading, writing, speaking, math, history (U.S. and worldwide), and overall accomplishment as a civilized young person -- to a typical undergraduate degree from about 1996 onward, Ivies included.
From what I've seen of the latest crop, the statistic is far too charitable.