BACKGROUND: The flexibility development issue has not been studied well enough. Still, the problem can be partially solved in physical education lessons in a secondary school if a set of simple exercises for flexibility development is permanently added to the standard physical education (PE) program. The study aimed to increase flexibility in children aged 9-10 years in PE lessons. METHODS: The research was conducted in secondary school number 40 (Kirov, Russia). The study involved 56 children from 3 RESULTS: For forward tilt performance, there was a significant difference in poststudy with EG resulting in higher performance than CG. Regarding post-to-pre research, EG and CG showed a significant difference with better values post compared with preintervention. Regarding the bridge test, a significant difference between groups was found regarding delta change with better performance for EG than CG. For post-to-preintervention comparisons, both EG and CG showed a significant difference with better values at post compared with prestudy. Concerning the boat test, EG resulted in higher performance than CG postintervention. CONCLUSIONS: If a set of physical exercises is introduced in PE classes to develop flexibility in schoolchildren aged 9-10, then their indicators will significantly improve.