The purpose of this study was to examine the effects of a short-term flexibility program on hamstring and lumbar extensibility and its posterior reduction among primary schoolchildren in a physical education (PE) setting. Forty-five 10-to-11-year-old schoolchildren from two classes were clustered randomly to an experimental group (EG)(n= 22) or a control group (CG)(n= 23). During the PE classes, the students in EG performed a six-minute flexibility program twice a week for eight weeks. Subsequently, these students underwent a five-week detraining period. The results of the two-way ANOVA showed that the intervention program significantly increased the students’ hamstring and lumbar extensibility (pretest= 15.7±7.0 cm; posttest= 18.2±7.7 cm; p<. 001). Although after the detraining period flexibility levels decreased statistically significantly (retest= 17.1±7.9 cm; p<. 001), the students from EG presented statistically higher values than in the baseline flexibility level (p=. 006). For the CG no significant differences were found (pretest= 13.4±8.5 cm; posttest= 13.1±8.5 cm; retest= 13.2±8.4 cm; p= 1.000). Although children lose a significant part of the obtained flexibility gains over a five-week detraining period, they do not revert to their baseline flexibility level. Hence, the students might continue working on their flexibility within the next five weeks in order to maintain the gains obtained previously. These findings could help teachers to design programs that guarantee feasible improvement and maintenance of children’s flexibility in a physical education setting.