High school students in the United States have been taking more challenging courses in recent years, but academic achievement has been stagnant. At the heart of the matter is the quality of curriculum, instruction, and assessment. Some courses tend to be more challenging in name than in practice. High schools also have a history of autonomy that inhibits the coordination of curriculum and the development of a common knowledge base for teachers. To improve the consistency and rigor of high school instruction, the National Governors Association Center for Best Practices (NGA Center) partnered with ACT, Inc. on a pilot project in Mississippi, Oklahoma, and Pennsylvania. ACT trained 98 teachers in 18 high schools on how to use state-of-the-art curriculum units and new instructional methods that were integrated with a system of assessments. The project focused on 10th grade courses (English language arts, geometry, and biology) and aimed to prepare more high school graduates for the demands of higher education and the workplace. The pilot project found strong indication that when high school courses are well-aligned to academic standards, growth in achievement occurs. The project also found a relationship between the level of implementation and achievement. The top implementing schools (teachers who attended all of the professional development and used the ACT units and assessments) made the greatest achievement gains. However, full implementation of the rigorous curriculum and assessment system was hard to achieve. Teachers need more assistance with blending rigorous instruction and new assessment practices. Additionally, school leadership must secure planning time to help teachers use new assessments effectively and efficiently. Project results suggest that states should consider three policies to increase the consistency and rigor of high school courses: (1) Ensure that high school courses are well-aligned to challenging academic standards and that course expectations are more consistent: (2) Include end-of-course exams in a comprehensive assessment system. Such systems can signal to students what it means to be ready for college entry and success; and (3) Provide teachers extended professional development to learn how to integrate new instructional methods with new assessment practices. (Contains 31 endnotes and 1 chart.)