To study the impact of the school food environment interventions motivated by the obesity prevention policy reform (Law No 1355, Colombia 2009) on body weight and diet quality trends of schoolchildren. Schoolchildren from six provinces that had implemented the law ('intervened schools'), and from 26 that had not, were studied using data from the National Nutrition Surveys (ENSIN). A total of 48,936 children aged 6–17y were included (2005, n = 16,193; 2010, n = 9,907; 2015, n = 22,836). Body mass index (log-transformed z-score; zBMI) and bodyweight excess (BWE; i.e., zBMI ≥ 1.0) were the primary outcomes. The Alternative Healthy-Eating Index (AHEI; log-transformed) was derived to measure diet quality, using data from single 24 h recall questionnaires administered in 2005 and 2015. A difference-in-difference analysis (diff-in-diff) was carried out using two waves of data before (ENSIN 2005 and 2010), and one wave 4 years after the policy implementation (ENSIN 2015). The identification strategy relies on variability across time and provinces. A total of 11,113 schoolchildren were in the intervened provinces (2005: 4,087; 2010: 2,861; 2015: 4,165). Overall, the percentage of children with BWE in 2005, 2010, and 2010 was 16.5%, 20.2%, and 25.0%, respectively. Between 2005 and 2015, the prevalence of BWE increased from 15.5% to 27.3% in the intervened schools, and from 16.9% to 24.4% in the non-intervened schools. The mean AHEI score was 46.2 ± 10.5 in 2005 and 43.7 ± 11.1 in 2015, with a decrease of 2.29 points in the sample of children from the intervened schools, and of 2.32 points in those from the non-intervened schools. The diff-in-diff analyses showed that the interventions were associated with a 4% increase in zBMI (P < 0.05), and a 2% increase in BWE prevalence (P < 0.001). The changes in diet quality were not statistically significant. Diet quality and trends of overweight and obesity in schoolchildren did not improve in the schools that implemented Law No1355. The observed decline in diet quality in all children studied suggest that implementation of food policies to promote healthy eating should not be limited to efforts centered in the school environment. GMG was supported by COLCIENCIAS (Fondo para Investigación en Salud-FIS-).