Individuals who have entered public school teaching in the 1990s and onward are less qualified for teaching than individuals who entered public school teaching in earlier decades (despite modest improvements in the past decade according to one study). According to a 2008 study based on SAT data, “education majors finished 25th in reading, 27th in math and a combined 57 points below the national average in both.” Teacher decline since the 1960s based on other earlier credible ability measures has been confirmed in multiple studies.
So, my simple theory (which I thought of independently but I am by no means the first person to propose this theory) is that the majority of public school* teachers—particularly at lower levels—have historically been women. The older (now retired or retiring) generation of women who chose to become teachers went in when the profession attracted more intelligent women. The teaching profession was a particularly attractive profession for women. Besides the inherent advantages to teaching (working with children, summers off, etc.), this attractiveness was due to a combination of traditional social norms and employer discrimination against women in other professions that intelligent men would enter. Teaching was one of the more prestigious and socially acceptable jobs for women to enter in the 1960s and 70s.