Does schooling affect social inequality in educational achievement? Earlier studies based on seasonal comparisons suggested schooling to equalize social gaps in achievement, but recent replication studies gave rise to skepticism about the validity of older findings. We propose an alternative causal design that identifies schooling exposure effects by exploiting (conditionally) random variation in test dates and birth dates for children participating in assessment studies. We test effects of school exposure in first grade for a series of learning domains (vocabulary, grammar, math, and science) by drawing on recent data from the German National Educational Panel Study (NEPS). Findings clearly indicate that schooling increases learning in all domains and particularly in math and science. However, we did not find any evidence that schooling effects differed by children’s socio-economic background. We conclude that, while all children benefitted by first-grade exposure, first-grade schooling had no consequences for social inequality in learning. We discuss the relevance of our approach and results in the context of the massive school lockdowns due to the COVID-19 crisis and to further knowledge on the role of schooling in the process of social stratification.