Standardized tests are a controversial part of the American educational system. They've become more popular recently as Americans have grown concerned about the quality of their children's education. Still, focusing exclusively on students' performance in standardized tests can be problematic.
In general, poor kids don't do as well on standardized tests as middle-class and rich kids. Consequently, while upper-class kids are studying history and art, lower-class kids are being drilled in test-taking. The result is an even bigger educational gap between the poor and the more well-off.
In the real world, it isn't enough to know how to add and multiply--you also need to know when to apply each skill. The key to real-life success is the ability to pull together different kinds of knowledge in order to solve complex problems. Most tests, including standardized tests, simply don't have the scope to measure these skills. So when you evaluate someone's potential, it isn't enough to look at his or her test score. You also need to look at how his or her performance compares to that of successful people in various professions.
Sometimes good students are poor test-takers, and vice versa. Standardized tests are hardly the be-all and end-all predictors of a child's ability and likelihood of success. In fact, studies show that the one thing that never fails to improve education is not increased spending on standardized testing, but increased spending on schools.