This is a hot topic in public education in the US right now - there's even a book called "The Dumbing Down of America." It is a result of many factors but the result is an inflation of school grades and students learning less and less over the years. For example, a test that was considered suitable for an 8th grade final exam in the early 1900s is now too difficult for a high school graduate (12th grade) to pass.
"Dumbing down" means to use simpler or easier language in writing or speaking. In America, there is much discussion that school books are "dumbed down" because American students do not read as well as they once did.
Dumbing down 
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