Actually, average scores on standardized tests have risen dramatically over the past 80 years or so.
If you look at the regents now, they have really been downgraded. Additionally, if too many students in one class fail, or get lower marks, they automatically pass many and upgrade scores (I believe its called a learning-curve scale. Not sure the name).
So while scores may be improving, “knowledge” certainly is not. I’m only talking about the regents. I wonder if its an indicator to other things as well, and studies have been slanted in a certain direction, as they are done at times.