The pupils were attending a private school in Peshawar.
As well as the female teacher, more than 50 other people were injured, a hospital spokesman said. Six girls were among the dead.
Several children remain trapped under the rubble and rescuers are trying to reach them, according to Bilal Faizi, a spokesman for the emergency services.
Most of the victims were brought dead to the Lady Reading Hospital, along with 20 others who were wounded, hospital spokesperson Mohammad Asim said.
Mr Asim said some of the injured were in a critical condition.
The hospital declared an emergency as medical staff treated the wounded.
The children are all believed to be under the age of 13.
The cause of the collapse was under investigation.
Peshawar police chief Saeed Ahmed said that the school’s owner had been arrested.
He said he feared the number of deaths could rise as just 47 of the 110 students in three classrooms at that time had been rescued.
“It was a weak building structure,” Mr Ahmed added.
“Its roof was overweight with construction on a weak structure that caused the collapse.”
There was no immediate comment from school officials.
The private Al-Khair Education Center school is located in Dir Colony of the northern city.
Television footage showed ambulances rushing to the scene and rescue workers retrieving wounded students from the rubble.
According to witnesses, residents were among the first to reach the school in a congested neighbourhood in central Peshawar.
Rescue workers arrived within minutes, according to accounts from the scene.
A resident, Mohammad Khan, said rescuers carefully removed debris in an attempt to reach children and staff feared trapped under the collapsed roof.
Outside the school and at the state-run hospital, anguished parents were seeking information about their children.
Building collapses are common in the country, where construction standards are often poorly enforced.
In July, a roof collapsed at a tutoring centre under construction in Lahore, killing four schoolchildren.
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