A short piece in the New Yorker gives this historical evidence:
“We haven’t ever known our past,” Sam Wineburg, a professor of education and history at Stanford, said last week. “Your kids are no stupider than their grandparents.” He pointed out that the first large-scale proficiency study—of Texas students, in 1915-16—demonstrated that many couldn’t tell Thomas Jefferson from Jefferson Davis or 1492 from 1776. A 1943 survey of seven thousand college freshmen found that, among other things, only six per cent of them could name the original thirteen colonies.The article also notes that many of these repeated tests drop those questions that most students get right in order to have ones that follow a bell curve. So maybe we're into a Zeno's paradox in tests of student knowledge. We'll never get smart enough.