Background: Hispanic children of immigrants are vulnerable to obesity and weigh more than their white peers. Theory suggests that school is a social institution that could foster weight gain among Hispanic children of immigrants and disparities in weight that emerge over time. Objective: We investigate whether Hispanic children of immigrants gain more weight during school years than whites and whether school year weight gains accumulate and contribute to differences in the weight of Hispanic children of immigrants and whites. Methods: We analyze ECLS-K:2011 kindergarten, first, and second grade data using descriptive statistics and multilevel growth curve models. We also calculate how the accumulation of weight gain during school years and summer breaks contributes to weight disparities between Hispanic children of immigrants and whites by the end of second grade. Results: Hispanic children of immigrants gain significantly more weight than white and Hispanic children with US-born parents during the kindergarten school year. The accumulation of weight gain during kindergarten, first, and second grade school years contributes to weight disparities between Hispanic children of immigrants and whites, but not differences between Hispanic and white children with US-born parents. Conclusions: Hispanic children of immigrants are vulnerable to weight gain during kindergarten. In addition, weight gain during the earliest school years helps to explain emerging ethnic/parental nativity disparities in weight. Contribution: The paper extends knowledge about negative health assimilation among Hispanic children of immigrants by showing that a primary social institution in children’s lives may inadvertently contribute to weight gain and emerging disparities in weight.