Thousands of schools across the nation are responding to the reading and math testing requirements laid out in No Child Left Behind, President Bush's signature education law, by reducing class time spent on other subjects and, for some low-proficiency students, eliminating it. Schools from Vermont to California are increasing — in some cases tripling — the class time that low-proficiency students spend on reading and math, mainly because the federal law, signed in 2002, requires annual exams only in those subjects and punishes schools that fall short of rising benchmarks. The changes appear to principally affect schools and students who...