In 1993, the Commonwealth of Massachusetts dramatically overhauled its K-12 education system and created a new school finance formula, building an educational accountability structure to ensure every child has access to a high-quality education. The Massachusetts Education Reform Act (MERA) established academic standards in core subjects, mandated assessments to measure student outcomes on those standards, and established a system for holding schools accountable when students failed to meet basic expectations. This system has helped Massachusetts' public schools become the highest performing in the country. Student outcomes in all tested subjects and across demographic groups have improved steadily over time, but disparities in achievement and attainment exist between the Commonwealth's most privileged students and their less privileged counterparts, many of whom are black or Hispanic. Without the MERA and its requirement to assess every student and publish aggregate academic outcomes, policymakers may not understand the extent of disparity or how to address it as student outcomes data are integral to understanding where Massachusetts' public schools have been, where they are going, and how they can get there. This paper illustrates the importance of the Massachusetts Education Reform Act and how it has positively impacted students over time. It explains why the current accountability system evolved as it did and why preserving the most important aspects of that system is critical if the state is going to fulfill its constitutional obligation to educate all children to a high common standard.