This quasi-experimental study evaluates the longitudinal impact of comprehensive school nutrition policy implementation on childhood obesity trends across 125 school districts. Using difference-in-differences analysis with 45,320 student observations over 5 years, we found that districts implementing strong nutrition standards demonstrated significant reductions in obesity prevalence compared to control districts. Policy effects emerged within 2 years of implementation and strengthened over time, with the most substantial impacts observed among elementary school students and children from low-income families. The findings provide robust evidence for school nutrition policies as effective strategies for obesity prevention.