The children found the bomb outside the school in Luqman Banda village of the district, which is part of the restive Malakand division, and it went off while they were playing with it on Saturday.
TV news channels reported that 13 children, most of them girls, died in the blast, while police officials confirmed the death of seven girls. They said over 40 children were injured in the explosion.
Those killed were aged between four and 12 years. This was the first incident of its kind after authorities announced that the Nizam-e-Adl regulation would be implemented in Malakand to enforce Shariah or Islamic law.
President Asif Ali Zardari and Prime Minister Yousuf Raza Gilani have condemned the incident.