This study was conducted to identify differences in growth based on the obesity index in sixth grade. Heights and weights of 141 students from first to sixth grades were collected from school records. Other information was gathered by survey, and the weight length index (WLI) was calculated. Subjects were classified into three groups: underweight (n = 57, UG), normal (n = 53, NG), and obese (n = 31, OG) using the WLI. Differences in weight, height, and growth velocity were compared among the three groups from the first to sixth grades. With regard to growth, the past physical status of the three groups was maintained. More than 50% of the OG was in the overweight range when they were in the third to fifth grades. Approximately 60% of NG was in the normal weight range between the first and fifth grades. More than 70% of UG was in the underweight range when they were in the first to fifth grades (p < 0.001). Growth velocity was faster in OG than that in NG and UG (p < 0.001). The annual growth rate of children was the highest when they moved from the third to the fourth grade (p < 0.001). The results indicate that general balanced diet education should be implemented in lower grades, because physical growth formed in lower grades tended to be maintained in senior grades. It would be better to educate students about diet to prevent obesity before the fifth grade when a major change in body structure has occurred.