I use standardized test scores from roughly 45 million students to describe the temporal structure of educational opportunity in over 11,000 school districts--almost every district in the US. For each school district, I construct two measures: the average academic performance of students in grade 3 and the within-cohort growth in test scores from grade 3 to 8. I argue that third grade average test scores can be thought of as measures of the average extent of educational opportunities available to students in a community prior to age 9. Growth rates in average scores from grade 3 to 8 can be thought of as reflecting educational opportunities available to children in a school district between the ages of 9 and 14. I document considerable variation among school districts in both average third grade scores and test score growth rates. Importantly, the two measures are uncorrelated, indicating that the characteristics of communities that provide high levels of early childhood educational opportunity are not the same as those that provide high opportunities for growth from third to eighth grade. This suggests that the role of schools in shaping educational opportunity varies across school districts. Moreover, the variation among districts in the two temporal opportunity dimensions implies that strategies to improve educational opportunity may need to target different age groups in different places. One additional implication of the low correlation between growth rates and average third grade scores is that measures of average test scores are likely very poor measures of school effectiveness. The growth measure I construct does not isolate the contribution of schools to children's academic skills, but is likely closer to a measure of school effectiveness than are measures of average test scores.