In this essay, I examine unmet promises and global challenges for achieving language equality in schooling, with special focus on one of the United Nations’ Sustainable Development Goals, SDG-4, which aims to ensure equitable and inclusive quality education and lifelong learning for all. Language of instruction is a key determinant of student success, but there is limited recognition of the vital role language plays as an intervening variable. Most languages continue to be excluded from education and 60 percent of out-of-school children live in regions where their own languages are not used at school. Inequities arising from unjust language policies combine to trap the poorest in a cluster of disadvantages persisting across generations. Underinvesting in education jeopardizes a range of social benefits. A well-educated population will increase the overall economic prosperity of a nation. I call for first language-based multilingual education as a pathway to schooling equality and sustainable development.