This mixed methods experimental study examined the impacts of the Early College High School model on students' college readiness in mathematics measured by their success in college preparatory mathematics courses in the 9th through 11th grades, and disaggregated for academically prepared and underprepared students. This study looked at the longitudinal sample of students who moved from the 9th through 11th grade both in the treatment and control groups. The results show that the reform is having statistically significant and substantively important impacts on students' course-taking and success for both prepared and underprepared students. The impacts of this whole school reform are larger for underprepared students. The results demonstrate that the ECHS reform model is being successful in implementing a universal algebra policy and rigorous college preparation curriculum with students of diverse backgrounds: 38% underrepresented in college minority, 46% low income, and 38% first generation college-goers, and despite of their academic preparedness levels. The analyses of classroom observations and interviews with mathematics teachers reveal that instruction in the ECHS displays a mix of traditional approaches and rigorous student-centered instructional practices. These results are discussed in the context of debates on benefits and disadvantages of universal algebra policies and student-centered instruction for academically underprepared students.