It reminds me of a post at D-Ed Reckoning tying lower performance with lower vocabulary attainment.
Primarily, the problem remains language acquisition. Language skills are especially difficult to acquire for low performers who (a) lack the cognitive ability to learn language quickly and (b) typically, do not grow up in language rich households. Most of the advanced vocabulary students will need to succeed in college will only come from reading lots of difficult books-- the kind of books that don't get assigned much anymore.
While I was reading Jane Eyre for book club, I noticed there were a slew of words that I had never seen before. It reminded me of all of the advice people get today for communication--never use a big word when a simpler one will due. I began to wonder if Jane Eyre was written badly because she didn't follow that rule. Perhaps a reasonably educated person of her time could understand it and modern writing and education is so bad that we do need to dumb down our communication.