Flexibility is an important factor of physical fitness to prevent sports injuries. However, the best timing to improve flexibility during the growth and development period in children has been unclear. To compare the acute effects of static stretching on flexibility between school-grade children in the growth and development period, we measured the hip joint flexion range of motion (RoM) by using straight leg raising (SLR) before and after static stretching (6 exercises, 15 seconds, 2 sets) in 50 young male children aged 7 to 13 years (the second grade of elementary school to the first grade of junior high school). The RoM in SLR significantly increased after stretching versus that before stretching in all subjects. This positive improvement of RoM was statistically significant among all grade levels. Notably, the change rate in RoM after stretching was significantly greater in the fourth grade of elementary school than in the other grades. The rate of height increase during the past year was lower in students from the fourth grade than in students from the second grade of elementary and the first grade of junior high school. These results suggest that flexibility training is more effective during the slow-down period of growth and development in young male children.