The aim of the study was to assess the effects that a steps/day programme may have on body mass index (BMI) among primary education students (11.37±0.48 years). A six-week controlled trial with a follow-up was completed with an experimental group (N=66, pedometer + steps/day programme + reinforcement programme in their physical education (PE) marks + weekly follow-up in PE), and a control group (N=76). Omron HJ-152-E2 pedometers were used. Normoweight students complied with programme requirements to a greater extent than their overweight counterparts (72.7 vs 59.1%). The programmed minimum number of 12,000 steps/day for boys and 10,000 for girls was exceeded by 83% of boys and 60% of girls. The differences in the number of steps/day between boys (14,274) and girls (10,626) were significant across all the measured periods (p<.05). The results show that the teenagers who complied with the programme requirements reduced their BMI significantly after the intervention (p<.001) and this reduction persisted for six weeks after the programme (p<.001). In conclusion, the six-week programme of 12,000 steps/day for boys and 10,000 for girls, jointly with a reinforcement programme in their PE marks and weekly follow-up by their PE teacher, reduces BMI significantly in 11-12-year-old schoolchildren. Monitored steps/day programmes in the PE curriculum increase out-of-school physical activity and reduce BMI in 11-12-year-old schoolchildren.