We use a quadratic equation to estimate trends in cohort performances in the charter and district sectors on the National Assessment of Educational Progress in 4th and 8th grade between 2005 and 2017. Data consist of over four million test observations of nationally representative samples of students on seven separate math and reading tests. We estimate unadjusted trends as well as trends adjusted for changes in the demographic composition of the two sectors. Compared to district cohorts, we find steeper upward trends in mean charter performance. Larger sector differentials are observed for African Americans, low SES students, and for students in the Northeast. No significant trend differences are observed for Hispanic and Asian Americans, suburban students and for students in the West. Since students in the two sectors may differ in relevant ways for reasons unrelated to demographic characteristics, inferences about changes in school production factors remain uncertain.