In my experience, if you're the smartest kid in your class, you're not smart. You're just in the wrong class.
Also, if you're the dumbest kid.
But I'll spot one further. Standardized testing exists to place students on a curve. You don't want everyone failing. You don't want everyone acing the exam. You want to be able to point and say "These are the good schools/students and the bad ones".
Coincidentally, the wealth, the politics, and the ethnic composition of the districts tend to speak far louder to exam performance. Schools that are targeted for privatization can suddenly find their students doing very poorly, year to year. Schools that have a partisan administrator with friends at Pearson can find themselves doing amazingly well, practically overnight.