This dissertation contains three essays about the economics of education, focusing on K-12 math education, college attainment in Science, Technology, Engineering, and Mathematics (STEM) fields, and merit-based financial assistance. The first chapter investigates the impact of stricter high school math curriculum requirements on STEM degree completion by exploiting cross-state variation in the timing of requirement increases on the number of years of math courses required for high school graduation from two waves of nationwide math reforms, the first in the 1980s, and the second in more recent years. I find that the earlier reforms that focused on adding minimum year requirements substantially increased STEM degree completion of traditionally underrepresented groups, particularly black and Hispanic males, and white females. By contrast, the more recent standards-based reforms disproportionally benefited white males, but had little impact on other groups. The results suggest that the effectiveness of math reforms on college STEM attainment depends on the specific features, and to effectively improve college STEM attainment for all groups, different policy interventions might be necessary. The second chapter examines whether the expansion of merit-based state financial assistance improved students' college outcomes. Merit scholarship programs provide state residents who meet certain academic requirements with grant money to attend in-state institutions. Taking advantage of the variation in program adoption dates and features across states, I find that the level of academic requirements and the amount of scholarship funding awarded are important determinants of program effectiveness. Specifically, the leniency of academic requirements largely contributes to associate's degree completion, whereas the generosity of scholarship amount significantly increases college attendance and bachelor's degree completion. In the third chapter, I examine the gap in college STEM attainment between immigrant and native students. Using detailed student survey data from the Beginning Postsecondary Longitudinal Studies 2004/09, I find that first-generation immigrant students, especially those who attended foreign K-12 schools, are significantly more likely to enter and persist in STEM fields compared to natives. The immigrant college STEM attainment advantage is particularly large among Asian and white students. I explore the factors leading to the immigrant-native college STEM attainment gap, including socioeconomic status, individual preferences, and academic preparation in math and science. Results from a linear probability model demonstrate that immigrants' higher college STEM attainment is largely due to better math and science academic preparation. The estimated immigrant-native differences in STEM entry and persistence are no longer statistically significant after including the full set of academic control variables. This suggests the important role of K-12 math and science education in improving students' college STEM attainment.