Take the guideline that most college-bound students should proceed through the K-12 math curriculum--a hierarchy of classes stepping from simple arithmetic to calculus--at a pace that lands them in algebra in ninth grade. This is too easy for most children. In a regular middle school two of my children attended, all of the students took algebra in or before eighth grade and over 80 percent mastered it, putting them one or more years ahead in math.
Entire countries of students accomplish this routinely. The Third International Mathematics and Science Study, conducted in 1996, found that the material taught in U.S. eighth-grade math classes was taught in the seventh grade in many other developed countries and even earlier in Japan and Germany. As a result, U.S. eighth graders performed significantly poorer on a standardized math test than eighth graders in twenty other countries, and far poorer than Japanese students, who scored highest. Overall, U.S. elementary and middle school math education lags a full year behind that in dozens of countries and one and a half years behind Japan and Germany.
Math Coach by Wayne Wickelgren, p. 4