The accident took place at about 7.50 p.m. on Monday when the school bus coming from Faisalabad city overturned on its way back from a picnic on the motorway linking the country's capital Islamabad and Lahore, capital of Punjab province.
A traffic police spokesperson said the driver lost control over the bus when he was taking a turn on the motorway near Kalar Kahar, a town located some 125 km south of Islamabad.
Most of the dead were students aged between 12 and 13, reported Xinhua citing media reports.
The school principal and some teachers were also killed.
Hospital sources said that at least a dozen people injured in the accident were in a critical condition and the toll could rise.
Initial investigation showed the accident was caused by the brake failure of the bus, said police.
Police said they will launch investigations to find out why the school management violated traffic rules to pack the bus with 105 students and teachers.