The problem is that we cannot accurately measure intelligence, and a person's intelligence is partially influenced by their upbringing and nutrition. For example, in nations with very poor nutrition, the brain does not form properly leading to low I.Q scores, but when those nations become richer, the I.Q of the population shoots up dramatically.
People aren't any MORE stupid than they were 400, 500, even 10,000 years ago. I can guarantee you that people in the dark ages were pretty ignorant. Genetically, we've barely changed over the past 10,000 years as a species.
And people aren't really getting "smarter" - well, technically they are because they have better nutrition and they can be educated at an early age, versus having poor education and having to spend almost your whole life foraging for fruit. But if you could take 1000 babies from 500 B.C and educate them and raise them in today's modern society, they'd be exactly the same as children today. Modern day people aren't smarter - just more knowledgable and refined.
There are some alarming trends - in the west (The US especially) many teenagers and children no longer care for science. As a result, over half of the researchers in the United States come from overseas, because so many American kids don't want to study math or science. The US is in a jam - it still has the best universities and the best scientific teams in the world, but it's not longer leading the way it used to - it's fumbling with the ball and it won't be too long until someone else snatches it from their hands.
A society gets "dumber" when it no longer respects intellectual pursuits. In the US, the religious right scorns science, popular media labels them as "nerds". The problem with so many teenagers today is that they think they can count on someone else to do their thinking for them. If they aren't careful, they could lose their position in the world of science.
So yes - in some societies, people are no longer being educated to the same degree. But they aren't stupid - just ignorant. And that's a correctable problem. All it takes is more education and better parenting.