According to the police, the explosion was caused by more than six kilos of explosives that were taken into a classroom in a plastic bag during a Koran lesson.
According to the Pakistani newspaper Dawn, more than 110 people have been injured, including teachers and several children.
The news agency Reuters lists 80 injured.
Five of the injured are said to be in critical condition.
The Taliban are distancing themselves
Pakistani Prime Minister Imran Khan has condemned the attack in a Twitter post.
"Deeply saddened by the attack on a school in Peshawar.
I apologize for the grief the relatives have to feel and I pray that the injured recover quickly.
I want to assure my country that we will ensure that the terrorists behind this cowardly attack are brought to justice as soon as possible ", writes Khan.
No group has yet claimed responsibility for the act, but the armed Taliban movement in the country has distanced itself from the act.
Recently, several attacks have been carried out in Khyber Pakhtunkhwa province, where Peshawar is located.