I can say that my 9 years of middle/high school classroom experience lines up exactly with Kohn's critique of a grades-driven educational system. Nothing annoys me more than hearing, "Mrs Ramey, will this be on the test?" as the prelude question to every lecture, discussion, discovery, or investigation.
As Kohn points out, kids are too smart to thirst for knowledge when their educational landscape is ruled by quantifiable expectations, benchmarks, percentage grades, and the like. I watch very smart students every day choose the easy way out because they see no reason to jeopardize their God-given leg-up in the achievement game. As a rule, I use a vast mix of assessments in my classroom if possible, and different kinds of projects or test quesions offer each student either challenge or relief once in a while. But it's tough to come up with great ideas all the time.
Parents and teachers must work together to combat the educational culture that ranks "achievement" and "success as measured by a number" as more important than effort, real learning, challenge, and curiosity. The most influential and effective agents of cultural change down through history have rarely been "good students."
Read Kohn's article. It's relatively short, and you don't have to be an education expert to grasp his point.
Cross-posted to Xanga