What is he talking about? IQ scores and SAT/ACT scores of students has generally gone up over the past 40 years when you look at individual demographics. It goes down slightly when you lump all demographics of students together because more low-income kids and kids from single-parent households finish school than they did decades ago. The white student dropout rate was near 10% back in the 80s and the black student dropout rate was near 15%. Now the white rate is down to 5% and the black rate down to 8%.
The fact that averaging all demographics together causes the scores to go down some is due to a statistical paradox known as "Simpson's Paradox":
However, US education scores have definitely gone down subjective to the scores of other industrialized nations in the past 30 years. That is due mostly to other industrialized countries' scores going up and thanks mostly to deregulation of school curriculum and standards (like the "No Child Left Behind" Act) & Republicans (and Democrats to a lesser degree) slashing education funds for the past 30 years to free up more money to spend on the military and corporate welfare.
China, one of the countries with the best education marks, spends 4% of its entire GDP on education. The US spends less than 0.8% of its GDP on education.