This report by the Center on Education Policy (CEP), an independent nonprofit organization, examines trends in the achievement of high school students on the state reading/English language arts (ELA) and mathematics tests used for accountability under the No Child Left Behind Act (NCLB). In most states, these tests are first administered in grade 10 or 11, although some states use end-of-course exams not tied to a particular high school grade. The authors looked at trends from 2002 (or a more recent year in several states) through 2009 for high school students overall and for African American, Asian American, Latino, Native American, white, low-income, male, and female students. Trends in average (mean) test scores were analyzed, along with trends in percentages of students scoring at or above the proficient and advanced levels of achievement on state tests. The authors also compared trends at the high school level with those at grades 4 and 8. Key findings from this study include the following: (1) Although high school students made gains in average test scores and proficiency in most of the states analyzed, fewer states showed gains at high school than at grades 4 and 8; (2) Many states show a troubling lack of progress among high school students at the advanced achievement level; (3) Achievement gains were smaller, on average, for high school students than for 4th and 8th graders; (4) States with gains for major subgroups of high school students far outnumbered states with declines, according to percentages proficient and average test scores. But at the advanced achievement level, high school trends for subgroups were less positive, especially in English language arts; and (5) Gaps between subgroups of high school students generally narrowed at the proficient level but often widened at the advanced level. High school grades tested for NCLB accountability in 2008-09 for states included in this study are appended. (Contains 8 tables.) [This paper was written with the assistance from Victor Chudowsky and Naomi Chudowsky. For related reports, see "State Test Score Trends through 2008-09, Part 1: Rising Scores on State Tests and NAEP" (ED513962); "State Test Score Trends through 2008-09, Part 2: Slow and Uneven Progress in Narrowing Gaps" (ED513914); "State Test Score Trends through 2008-09, Part 3: Student Achievement at 8th Grade" (ED518144); and "State Test Score Trends through 2008-09, Part 4: Is Achievement Improving and Are Gaps Narrowing for Title I Students?" (ED522866).]