Background: Body mass index (BMI) increases when students are away on summer vacation. Evaluation of serial BMI measurements on year-round students allows new insight into the reasons children gain weight seasonally. Methods: The 206 first and second graders of 2-year-round elementary schools with obesity intervention programs were weighed and measured every 6 months from April 2011 until April 2013. BMI and BMI percentiles were recorded. Results: Children <50th, 50th to 85th, and 85th to 95th BMI percentiles showed significant accelerated winter gains in BMI (p = 2.6 × 10[superscript -7], p = 6.8 × 10[superscript -10], and p = 0.0007, respectively) compared with the previous summers. Accelerated BMI growth of the second summer was absent when compared with the previous winter BMI changes in children <95th percentile (p = 0.48, p = 0.81, and p = 0.93). For children >95th percentile, BMI gain continued without significant changes year-round. Conclusions: School-wide interventions to induce BMI growth limitations are associated with significant rebound BMI gain with possible wintertime augmentation in nonobese children of year-round education programs. Obese children do not display rebound BMI gain, BMI seasonal change, or BMI limitations in response to school-based interventions. School weight control programs should consider off-campus sources of weight gain and be vigilant for rebound and delayed seasonal BMI gains.